Seiko STH-8BLD-3 has a low-speed rattle and I'm stumped

[saja boi]

I need some advice on my Seiko STH-8BLD-3 compound feed machine because it’s making a really bad, low-pitched rattle or groan when I sew slowly and the noise totally disappears when I speed up. I use this machine for making backpacks and have owned it for about a year now.

I always keep it tuned and oiled, but this noise started slowly and oiling it again did not help at all. I have checked everything I can think of, including swapping out the servo motor and checking for loose parts like the hand wheel and sensor. But the machine still sews perfectly fine.

I think the sound is coming from the enclosed area near the upper oil reservoir, but it is hard to tell. I tried a mechanics stethoscope, but the noise is just too loud to pinpoint the source. I fix my own machines because I do not have a local mechanic, but I've spent two nights on this problem and I am out of ideas.

Could it be a gear with a slight rough spot that only makes noise when it does not have enough speed and load? What about checking the gear meshing clearance in that enclosed area?

 

[purple bean]

I could be wrong but it could be gear meshing issue or a slight problem with a bearing that is only noticeable under low-speed... high-torque conditions. Since you hear it near the upper oil reservoir, check the gear meshing. Possible that a tiny bit of wear on one tooth flank or a slight change in the gear clearance is causing a momentary bind or rattle when the slow movement is not enough to overcome that little rough spot smoothly. Try removing that top cover to inspect the gear teeth for any scuffing and check if there is any play in the shaft bearings that support those gears.

 

[WindWhisperer50]

@saja boi before you dive into the gears, try this one last non gear check, especially if you think the sound is high up. first, check the take up Lever connecting rod/eccentric. This part moves a lot. Make sure the bushings/bearings on the eccentric connection are snug. If there's any vertical play, it'll rattle like crazy at low speed.
hand crank slowly (Power Off!). put a little tension on the drive belt and slowly turn the handwheel. Can you feel any notch, click, or rough spot corresponding to a gear tooth coming around? If so, you've confirmed your gear theory!