We have a rheem furnice that constantly hums. The motor will turn on and off like it should but still hums when off.
Normally when a motor hums it is stuck but not this time.

Transformers can also cause humming noises and vibration on the body of the unit. Have you checked that or tried putting a glass half full of water on the outside and see what the vibrations look like? (do not pour out the water )