Changed out a good drum-roller (had a new one on-hand) on the side making the noise - still very noisy.
Thought it might be motor bearings, then. Tested the motor with old blower wheel and belt removed. Motor hummed along just fine. (Still worried it might only happen under belt load.)
The old blower wheel was kind of ate up AND had rubbed the plastic housing next to the motor.
Put in the new blower wheel and still had some noise. Not as bad; but, not good. My hope was it would just wear off the rough spots on the housing and/or shaft and settle down.
Success! After a few loads the noise was gone. Just the regular humming of the machine.