Are you sure that's not the meter arm snapping back into position? With a lens mounted, the last few degrees of wind arm travel puts the meter arm back into position and it goes 'boing' when it happens..
See if the sound goes away with no lens on as you wind.
On one of my M5 a screw near the base of the arm pivot had gotten loose and the arm scraped a baffle as it went into position, sounding and feeling awful. I think that was probaly why that body was up for sale in the first place. A quarter turn of a screwdriver fixed it.
You can see the arm in action if you put an LTM adaptor on then unscrew the lens, leaving the adaptor to press the lens detect lever at the base of the bayonet mount.