Quote Originally Posted by maxspongebob View Post
Let me think out loud for a second.

If the motor with a heavy pulley installed makes more noise than the motor with a light pulley, what is the difference?
What could cause this? Pulley out of balance causing a vibration in the motor internals?

If both pulleys are tight to the shaft, then a noise difference must be generated internal to the motor.
Out of balance could cause the motor bearings to chatter. Out of balance at high RPM could cause excessive motor bearing loads. Adding more load with belt tension may make situation worse.

Does the motor without a pulley make any noise? It shouldn't.
Does a radial load applied to the motor shaft make a noise? (how to test?)
Is there run out on the motor bearings?
The motor without a pulley sounds as you'd expect it to. I don't know how/why the cast iron pulley would be out of balance since it's brand new and made for the shaft, it's not custom like the one I have to open the bore for the spline shaft. It's possible that it's only rated for a certain RPM which is well below 3500.