Sounds like an EMC issue to me. Poor grounding of the electrical equipment on the aircraft causing potential differences between them. It could also be something simple like a poor ground plane between the ae and airframe. It could also be a bad transmission line (coax cable). Actually the causes could be extensive but I would start with the bonding between electrical components on the aircraft to be honest and work from there. A competent aircraft electrician should be able to help. I doubt the actual radio transrec is unserviceable.
Just my 2c worth (I am an ex-military helicopter avionics/radio tech).