[mbenonis]

Van, I've always wanted my toilet to double as an AM radio!

Seriously though, replace the mic cable with a new one and see if that fixes the problem. If not, run good mic line directly from the mic to the console, skipping any in-wall wiring. Continue to simplify the system until the problem disappears - when this happens, you've found the culprit.

Any sound system located near a powerful source of EM radiation, such as an AM radio transmitter, should have properly shielded and balanced wiring at all points in the system, as well as properly designed equipment. Even the smallest hole in shielding, or unbalanced run, can act as a wonderful antenna for that AM signal, and the solid-state devices we all use today act as rectifiers for the signal, injecting it directly into the audio path. THe only way to avoid it is shielding, shielding, and more shielding.