I bet you have a ground that not a ground. Start looking in your low level audio stages first.
does the rig have hum in cw mode? if so, you know its not audio stage specific. slide it in there op it on cw and see how your cw note is. If it is hum modulated it has nothing to do with audio. bad grounds, rusty hardware.
he does have a 3 wire grounded plug on it,
right?? never ever operate any of this old crap on a 2 wire cord. Good way to die given the right circumstances.