karl mcgruber wrote:

Take out the ELT and see what happens. Sometimes they will cause
problems. Mine did.


Did it, no change. I strongly suspect that it is a new noise getting in
by way of a fault (perhaps in the ground system) that has always been
there. Looks like there are two choices: rewire the stack going by the
install manual for the KMA-24 to hopefully eliminate the entry point
into the audio system, or find the source of the noise and eliminate it.
Neither is particularly attractive, which is probably why the avionics
shops don't want to touch it.