Oooh ok!
First thing I would check is to make sure the fan sheave has a small space between the end casing and the bottom of the sheave. There is a small shoulder on the bottom of the motor shaft. The sheave should sit all the way down on that BUT above the casing. Sometimes this shoulder can get screwed up.
Start by removing the sheaves see post #3
here
Check the shoulder, it should like like this:
Check the shoulder, key and key way, and bottom of the fan sheave. If all look ok, run the motor without the sheaves (don't leave the key in the shaft to become a projectile). If the noise is gone you know it's not in the motor but in the sheaves. If the noise is still there, stop and let us know as that will take us in a different direction.
If the noise is gone without the sheaves, reinstall only the fan sheave and run the motor. If the noise is still gone, reinstall all as shown
here