Comm noise

I flew yesterday with an owner who pointed out that he is getting a
type of crackling noise over both comm radios. The noise is most
prevalent when receiving from more distant and weaker transmitters
like other aircraft and not noticeabl when receiving from closer more
powerful transmitters i.e. the tower.

The owner has operated the aircraft with every other system turned off
including the alternators and has operated on individual magnetos with
no change. The most recent change to equipment was an audion panel
installation. The noise is not rythmic like ignition noise and does
not rise and fall with engine settings like some alternator noise. It
is not as intense as the white noise when checking squelch. It is a
type of random crackling like noise.

Any thoughts?

John Dupre'