Generally, buzz could mean 120Hz, hum 60Hz.
120Hz would mean there's a malfunctioning power supply somewhere- faulty electrolytic? bad rectifier? because there's a bad ripple on the DC somewhere...
I would also check the power waveform on a 'scope, if it deviates from a sine too much that could invoke problems.
If you ain't got one, buy one from all that money you save...