Had a simular problem with a Giles 202 and Airtronics radio. Someone had mentioned to look for metal to metal rubbing but as I found out grafite to grafite will do the same thing. Are you running grafite pushrods? The problem with mine turned out to be the antenna wire would pull down under a g load (loop) close to the rods as I worked the elevator it would generate enough frequency to interfere with the radio. I ended up running the antenna along the bottom inside of the fuse and secured it with a little tention and the problem no longer exsist. Good luck and hope you get down to the problem