Midway Industrial came over to look at the problem

and found that the overhang weight was too much for the mount. There is no support for the motor except the mounting flange of the gear reducer and all of that weight is held by the 4 small bolts and the small footprint of the gear reducer that is attached to a shelf behind the drum. The noise stopped immediately as soon as we leveled the motor. Shims were not the answer. We are temporarily supporting the back of the motor with a piece of 1×2 that extends to the floor to level the motor and take the pressure off of the gear reducer, which we are still going to replace with a Grove.