[JRTJH]

Locate your power distribution center. One of the 15 amp circuit breakers will be listed as "CON/REC" or similar. That is the converter power breaker. Turn it off. If the "humming" goes away, it's either the fan in the converter (it is speed regulated and only comes on under load, as the load increases the fan speed increases, so the "humming" would get louder) or it's the transformers inside the converter. My guess is that on a 2006 model, there's significant dirt/dust/hair clogging the fan entrance grill and/or the fan blades. That debris is causing the fan blade to wobble, creating a "humming sound". The other possibility, the transformers inside the converter are possibly loose and/or the insulation has broken down enough that the magnetic field has increased sufficiently to cause vibration at 60 hz when power is applied.

Turning that CON/REC breaker off should stop the humming. If it does, locate your converter and investigate the cause. I don't know when Keystone switched from the old Magnatron brand converters to the more modern WFCO converters, but the Magnatron brand always "hummed" even when new. The humming got worse as they aged.