Suspiciously like a classic grounding/shielding issue. Some of the cabling in older types can be crummy at best and all it takes is a badly munched connector or a split coax to introduce all manner of bizarre problems that will have pilots and engineers alike scratching their heads furiously.
Bad earths and non-existant or crap shielding are common and cause all sorts of weird noises on radios. The number of folks who fly around in aircraft who's avionics exhibit all sorts of bonkers noises and yet think nothing of it is quite astounding.
Smithy