Yesterday I installed 3 Marley electric baseboard resistance heaters in my finished basement: a 2500 watt, 1000 watt and a 750 watt unit. All are 240V running off one 30 amp double pole circuit breaker with 10 gage copper wire. Each heater has it own thermostat. The 1000 and 750 watt units have bi-metallic manual thermostats, and the 2500 watt unit has a double switch digital electronic thermostat with setback.

The 750 and 1000 watt units work fine. The 2500 watt unit emits a low buzz or hum when it is running. It is in my home theater area of the basement so the sound it makes is objectionable. Any thoughts what could be causing this?

I tried to dampen various parts/areas of the heater with my hands, but was unable to determine the source of the sound. Please help!