I live in Florida and I have a 12 year old Janitrol unit. The indoor air handler unit has developed an intermittent humming sound. The unit works and the fan operates and seems to spin freely. When the humming develops turning the fan off does not stop it. Only turning turning power off at the CB will stop the humming. And when the power is reapplied the humming does not instantaneously start up. It may start a little later or not. It does not seem to be tied to the fan operation. The humming is accompanied by a slight vibration off the fan housing. The only other thing attached to the fan housing is a capacitor (12.5 uf, 370 VAC). Could the cap be the problem?
Thanks.

It could be the transformer plates are a bit loose. Many are like that. If you feel it is the capacitor than have it tested.