Yesterday, I had an ac change-out and installation of a new Trane 3.5 ton straight ac outside compressor unit 4TTR3042A, and indoor air handler unit 4TEC3F42, which replaced an 18-year-old Reem. The new Trane works fine, except for a hum coming from the air handler unit, that's audible in the livingroom when the unit is off (turned off by the automatic temperature control), or with the "heat -off - cool" switch physically turned to "off."
There was no air handler hum when off, with the old Reem unit replaced, or with other central ac units I've owned in other homes in the past.
1) Is the hum with the new Trane unit caused by a poorly wired hookup? 2) Is the hum caused by an incompatibility between the air handler and the White Rogers thermostat installed? or 3) Is the hum caused by a poor design in the Trane unit wiring itself, that supplies electricity directly to a transformer, without going through a relay that's controlled by the thermostat?