A few more details would help a bunch.......F-N-R, rpm, etc.
If you are talking about in gear, at idle (up to 1K RPM), I'd suspect the damper plates are worn a bit or the engine hasn't been tuned in a while. Its a common 'feature' on the older drives, too.
If they make the noise all the time, irregardless of gear selection, it could be the plates but more likely something inside (not good).
Many of the early models had issues the velvet drive attributed to poor maintenance by the owner. May wanna check the oil level and look for contamination.
BTW, I think those are "down angle" gears, not "in-line" (old style) gears.