We had a Hudson from the latest batch returned by a customer complaining of excessive noise when muted. We disassembled and found the problem to be the foward motor stop (see pictures below).
The motor had moved foward to this stop and pitched slightly upward. This caused the worm to rub on the inside of the gear box cover making noise.
To solve the problem we installed a piece of small shrink tube (white in photo) over stop. This prevented the forward motion of the motor just enough to solve the problem. Also there was no lubrication on the gear box. We had another loco with a noisy tender which also had no lube on tender.