Ceiling fan humming

I have a ceiling far that is producing a humming noise that seems to be coming from the motor housing. I followed the advice in the latest issue and replaced the capacitor and the switch, to no avail. I had tried replacing the wall switch as well (the fan and light are switched separately). After this did not work I replaced the entire fan with new. The first night was great, dead quiet, the second night the same noise is starting to come back. This is not an obnoxious hum and would not be an issue except for the fact that it is directly over our bed. I'm beginning to wonder if there may be an issue with the wiring that might cause a humming? Is this a possibility?  It may be a long shot but I'm hoping someone might have some insight, or other things to try. I'm comfortable working with wiring once I've cut the power but unfortunately don't have enough know-how to trouble shoot this one. Thank you in advance for any input. 

mdnyrx:

This is not an obnoxious hum and would not be an issue except for the fact that it is directly over our bed.

You have given the most likely cause in this statement.  That is, the place where your fan is, is quiet, and you spend time enough there for your ears to adjust to the quiet environment making then more sensitive to normally quiet sounds.

I suspect that your fan is not a DC (direct current) motor.

The more economical fans have AC (alternating current) motors that rely on a constantly changing magnetic field that regrettably creates vibrations in the metals of the motor body that can't be physically or electrically dampened out. 

DC motors simply don't have this vibration problem.  The electronics that provide the conversion of the AC power supplied to the DC fan, if of sufficient quality, will also be quiet. . . All of which comes at a higher price.