I once constructed a "electric heater", which was basically a large DC supply (just a full bridge and a filter cap) shorted across an array of wires. I observed something interesting and I just wanted to make sure my explanation is correct.
These were my observations at low and high power.
1. When I ran the heater a low power (with the wires not very hot) there was a distinct buzz. I assume this buzz was caused from the wires, and was due to the 60 Hz ripple that my load placed on the DC supply.
2. When I ran the heater at higher power (with red hot wires) the buzz nearly disappeared. Once again, at this power level the wires were red hot, and the heater was consuming about 1000 Watts.
I think what caused this is simply the fact that when the wires became red hot, their vibrations were dampened, because they were becoming more malleable, or moving towards the liquid state.
Also, I believe the 60 Hz hum is caused because the wires carried a magnetic field that varied in intensity. (the ripple in the DC supply) This variance of the field causes them to attract/repel eachother at a distinct frequency right? Similar to guitar strings?