I just replaced my bathroom fan motor, which had become very noisy. The new motor is much quieter, but still produces a knocking sound at about 90/minute at even intervals.

Is it possible that the motor magnets are unevenly weighted around the center? In any case, tightening the motor-wheel assembly as tightly as possible inside the casing and against the air exit outlet (going to the outside) reduced the sound a bit, but it is still noticeable.

What is the possible cause, and what do I need to do to remediate the problem, short of installing an entirely new unit?

Thanks for any comment

What brand fan and did you replace just the motor?

It is possible that dust and dirt have accumulated on the fan assembly. Since you took the motor out, you know how. Do it again and see if the blade or cage is clean. Just a small amount of collected debris can throw the fan off balance and cause the assembly to vibrate. It is also possible that the cage, if it is plastic, has a broken piece.

Nutone QT-110N-B is the appliance model and yes, I just replaced the motor and cleaned the whole thing. Thanks.

Thx-everything was clean when I re-installed the unit. This morning when I got up no knocking sound was noticeable. On your advice I will keep an eye. I hope that was not wasted money, since the old motor still works-is it difficult to recondition?

Those are typically small induction motors, /20 HP or less. The only thing you can do is to clean the windings and there should be a small oil hole in the end of the armature case. 10 wt or 3N1 will work.