[1000mp] Hum in Monitor Circuit

[Bill]

I've noticed my standard three-year-old MP has picked up a hum in the
transmit monitor circuit on SSB, AM and to a lesser extent, on FM.
It sounds like an a.c. hum, and is present when I key the rig with our
without a mic attached. Its loudness increases as I increase the AF gain
setting.  The hum is not evident when the AF gain pot is fully turned
counter-clockwise.
Again, the hum is present whether a mic is connected or not.
I would appreciate knowing if anyone has run across or heard of this
symptom, and what the remedy might be.
Thanks,
Bill