Soft "Thump" Sound During Preheat

Over the last few roasts, I’ve started hearing a regular “thump” sound as the drum turns during preheat. It’s actually not very loud, but I can feel the vibration of the thump through the table. The drum seems to be spinning fine and during actual roast I don’t notice anything. Should I be concerned? What’s the best way to troubleshoot?

I think I hear this too and I believe I saw others attribute this to beans getting partially trapped between the drum and the door, or the front of the roaster, so that either the door moves slightly or the drum actually moves slightly. While I’m not sure what is specifically causing it, I do know that I ignore it with no ill effect.

Thanks for the response. What I was hearing was more of a very soft “thump” noise during preheat, so there were no beans in the drum. I’m aware of the noises caused by beans near the door.

In any case, I haven’t heard the noise in quite a while so I think it’s ok. It might have been some chaff stuck under the drum or something. If anyone else has run into this though, I’d be interested to hear your theories on what it might have been.

The motor pulley can get loose, which is another thing that can cause the drum to thump. If it’s persistent you can tighten it, Aillio has a pdf somewhere that shows you how. But if it’s just once in a while I wouldn’t mess with it (and I also get those, maybe once every 5-6 roasts).

Edit: just reread your preheat comment. Maybe the motor pulley is a bit tighter when first starting up because of the ambient temperature, and pulls too hard on the drum, but stretches a bit as the roaster warms up and relieves the issue? Just a theory.

1 Like

Thanks for the ideas. As an update, I’m no longer hearing the thumping noise during preheating. I’m thinking it did have something to do with a cold roaster warming up, thanks for the suggestion @1WithABullet! My exhaust setup allows colder air to float back into the roaster if I don’t block the ducting, and I’m guessing things were just cold those times I was hearing the noise.

1 Like


I am dredging this up from the dead, because I believe I am having a similar issue with my new Bullet. I am getting a thump or clunk, exactly once per drum rotation (made an index mark with a paint pen on the front of the shaft), but only when the machine is hot (rather than cold as for the OP), which can be pretty clearly felt through the body of the machine. This occurs late during preheat - no beans in the machine - and continues during roasting and afterwards during cool down, but is hard to hear over the sound of the beans in the drum. It comes and goes in intensity, sometimes disappearing entirely, with no clear pattern or periodicity to those “epicycles” and is worst when the machine is hottest. It has continued through several roasts, so it’s hard to see how this can be a bean, which would have been turned into dust by now. Obviously, being only and exactly once per drum rotation, it can only be from the bearings, drum, spring, or drum pully: nothing at the motor can do this.

Here are some notes I sent to bullet support at SM. Their reaction (after seeing/hearing a video also) has been that it doesn’t seem serious, and yet I cannot shake the sense that something is not right and that damage could result.

Further observations:

  1. The sound definitely seems to be coming from the back of the drum someplace. Being 1/rev is really has to be the drum or something directly attached to it: the drum pulley, a bearing, or the spring at the back of the drum.

  2. After a couple more roasts, I’ve noticed that it doesn’t make this sound when cold. It only shows up when close to fully preheated, and it remains until the roaster is pretty much fully cooled. It comes and goes in intensity - sometimes disappearing entirely - throughout the roast. It’s harder to hear when there are beans in the drum, but it’s still there, and still 1/rev of the drum, so a lot faster during roasting. It is actually easier to put your hand on the machine and feel it during roasting as a mild thump through your fingertips.

I actually think the sound is likely the same as this report - except I hear it when hot instead of when cold - where there was no clear resolution:

Soft "Thump" Sound During Preheat


  1. I did a careful check for beans at the front of the roaster near the door. Nothing. Still, this would have been surprising to give a clunk noise at exactly 1/rev, and through many roasts. Any bean in there would be dust after 2-3 roasts. I also considered a bean fragment that got out the back, but again - anything like that would have been charred and ground to dust by now.

  2. I opened up the back to look around. Because it is exactly 1/rev, it really cannot be anything related to the motor mount unless the drum pully is slightly eccentric (or mounted slightly off center because of the set screws?) so that it pulls on the motor a different amount at different drum angles. However, looking at this more carefully, I don’t think this can be the cause. There is no visible motion and the motor seems to pivot smoothly anyway.

  3. I checked the set screws on the drum pulley and they are fully snug to my fingers, in consideration of being threaded in Al.

  4. I’m not sure how tight the bearings for the drum shaft are in their bores. I suppose they could be shifting around as the drum rotates. I’m not sure I buy this either. The bearings seem to have very little free play in their bores, and I don’t think higher temps can explain very much change in free play over such small length scales (diameter of bearings is small).

  5. I’m not sure what the end of the drum spring looks like and what it might contact as it rotates. In principle, it rests only on the inner bearing race, and cannot be touching anything or binding/releasing as the drum rotates as long as that bearing is rotating freely. But this does seem like a possible place for a 1/rev sound.

  6. During the last cool down, it was still making this noise some. I opened the chaff collector to observe and saw nothing notable in this area and with the best light/view I could get at the back of the drum.

  7. There are no signs of rubbing, distress, or misalignment at the back of the machine.

  8. The drum pulley is mounted with very tight clearance to the back of the drum enclosure. Although I see no sign that it has contacted the machine, this is further forward than I would personally mount that pully considering unknowns in thermal expansion of different components during roasting, and I think it could be safely moved rearward a small amount without risking the usual rubbing problem against the motor frame.

So… any ideas or wisdoms out there? I’m not really eager to remove the drum to look for signs of contact given the degree of disassembly required, but whatever is going on seems to be out of sight with only modest disassembly.


If you haven’t done it yet you might want to start a trouble ticket with Aillio. I have a hunch they’ve dealt with the issue before and can guide trouble shooting.

My roaster has a rhythmic noise during preheat… a scraping sound that repeats once per revolution. Unlike your ghost, mine goes away when the Bullet comes up to roast temperature. No clue what causes the noise but I assume it’s related to the drum expanding with rising drum/roaster temps and I don’t feel like it’s a threat.

In your case I wonder if this is related to the drive pulley on the rear of the drum. I’m going to presume my roaster is set up properly. I’ve never tried to adjust the belt in the 2 years I’ve had it. So my guess is nothing more than speculation. The pulley on my roaster slides on the drum shaft; not freely because the belt restricts movement, but it’s not firmly tightened to the shaft and can move to allow some freedom to align. I wonder if the drive pulley on your roaster is perhaps tightened too much… ?? [The Bullet drive belt tensioner has been redesigned since I received mine and I have no idea if belt-tension setup is different.]

Another thought is the possibility of a flat surface “oil canning”. The only (sort of) flat surface I can think of is the rear plate of the drum. That’s pretty studly and I doubt it’s the culprit but… ??

Aillio has probably heard of this before so I think that may be the best place to start.


Thanks. FWIW, the two-set-screw attachment for the drum pulley is not a design meant to be left loose, and my professional opinion is that nothing good can come from that. In fact, the manual describes a procedure for setting the position of this pulley that makes it quite clear that the pulley should be tightened on the shaft.

I’ll report back what I learn.

Hey all!

I’m also new with my Aillio bullet V2, so far I have roasted 6 times in it, and also started to hear a click sound kinda like a soft sound of a metronome, it’s every 2 seconds, but once I’m roasting it goes which is probably because of the sounds of the beans inside. Also checked every screw, looked for beans stuck on any crevasse possible and checked the pulley, as per instructions the pulley is 1mm apart.

Even on my last roast as it was cooling the sound continue, there is no other sound, no cringe, screech. The drum seems to be rotating perfectly and everything else works perfect too, just that clicking/metronome sound.

I’m at my wits end, to keep troubleshooting and finding a solution.
Hopefully some of your experience and knowledge could work on my bullet.

Hey again. I did get in touch with Aillio as suggested and got some very attentive help although it does not sound like they have seen this problem much. The suggestion was to remove the drum and inspect, which I did. I found nothing, but after re-assembly it didn’t make this sound on a preheat test. Message I just wrote to Aillio support below:

First off, thanks for your very attentive help. It’s really appreciated. I took a break from work this afternoon to dig into this. I started out by setting the machine up on an incline (2”X4” under the back feet), starting a preheat, and then carefully vacuuming around bottom of the drum to get as much stuff out the front as possible. Then I set it level again and let the preheat go to 200C. Almost exactly at 200C it started clunking as before, so I cooled it down before opening it up.

I did end up making a simple support for the front plate by cutting a slot in a box of the correct height for it to slide into which can then be swiveled to keep it out of the way (photo). With the heavy handle removed, this is enough to support it pretty well. It does take a moment to realize there are a few extra cm of wiring harness inside the machine that allows the front plate to be swiveled out of the way. If I had a block of wood large enough, I’d throw it on the table saw and make a slot that’s just right to hold it even more stably, but maybe another day.

On the backside, a quick inspection shows that clearly removing the frame is the way to go even with good ball-end tools. The hole/slot arrangement on the pins is good engineering. The screw that now gets left out because it’s inaccessible from the exhaust tube not so much, but that eighth screw surely isn’t needed. New connectors are always stiff to get apart with big fingers - especially the longer header - but patience always wins. I found something to support the rear frame too, instead of cutting the zip tie for the motor wires (I don’t have little ties here at home.)

Anyway, I found no signs of distress anywhere. I checked all the mating surfaces and clearances. The only signs of any contact with the drum look like something left from manufacture (photos). These are really tiny marks: mm-scale. There are no signs of any contact with the machine. There was some chaff still near the back of the drum, but nothing at all hard in there. I vacuumed that out and re-assembled before doing another preheat test.

I figured one of two things would happen. If it still clunked, probably the shape of the drum is creating some stress when it expands that causes it to pop from one configuration to another as it rotates. If it didn’t it means I changed something about some mating surface that stopped it from shifting back and forth during rotation. I let it equilibrate all the way to “Charge” and it didn’t make a single sound.

I do have a hypothesis. (I’m a physicist… it’s an occupational hazard.) Inspecting all the bits as they came apart, the only thing I saw that was odd was the set screw mark on the square key. Typically the set screw should leave a circular mark where if it is tightened down perpendicular to a surface. However, the mark on the square key was no more than half a circle. Meanwhile, I noticed when I put the pulley back on, that it was possible for the pulley to be rotated quite a bit relative to the shaft and doing this cocked the key slightly in its groove. I suspect if you tightened down the other set screw (the one that goes directly to the shaft) before the screw that goes into the key, the key can be left slightly cocked and the coupling of the pulley to the shaft isn’t really as secure as it should be. With a big change in temperature, the aluminum pulley is going to expand more than the steel shaft (Al having a larger CTE) and that could cause the pulley to begin to shift back and forth on the shaft. In any case, I really think this coupling is the most likely place for something funny to happen that would cause something to shift back and forth and make a noise. When I put it back together, setting the pulley positioning and tightening the set screws was the last thing I did, and I was careful to tighten the set screw into the key first.

If I could go back in time, I would probably have tried loosening both set screws, rocking the pulley back and forth to make sure it was free, set it to a neutral position, and re-tighten making sure to tighten the screw into the key first. As it stands, unless it starts making noise again, I’ll likely never really know for sure, but the above is my best guess.

In any case, all is well, and for sure there is no sign I was doing any damage.

7Drips: if you put a pen mark on the end of the shaft, pretty sure you are going to find it’s once/rev. Also, you will be able to detect it better with beans in the drum if you put your hand on the door handle, where you can feel the vibration very acutely. Hope my experience helps out. If nothing else, I can assure you Aillio is going to have heard of this before.

1 Like

Thank you so much for your input!
Tomorrow I will have the day off, so going to check again the back of my Bullet and write to Allio to asset this sound if things don’t change after checking it.

I accidentally ended up making a second user “tknightnelsonzpll” via Google login when I posted these and thought I should claim these posts in case there are followups later since I’m going to kill that account. 7Drips… I hope you got this figured out. I’ve heard no more noises after several more roasts.