1995 mach 460 issues

i have a 1995 gt with the mach 460 stereo, when i start the car i get a loud humm noise from all the speakers, no noise when the key is turned back, this noise changes when i turn on the headlights and changes slightly with rpm aswell,
i am pretty sure its a bad ground but i was wondering if anyone knows where i shoud start looking or what i should do to fix it,
thanks in advance