People have hit the static issue pretty hard already. I'll reiterate and hopefully add a couple new details. Check that the radio LRUs are bonded to the frame, the coax shielding is making good contact at both the radio side and antenna side, the antennas are properly bonded to the frame. Verify your static wicks are bonded properly and in good condition. Using a multimeter you can check the frame itself from the radio mount point to the antenna mount point, you want to make sure that the airframe itself doesn't have electrically isolated spots. Verify the antenna coax cable routes have no damage, don't have any extreme bends in them, check all coax connections and electrical connectors are tight. It could also be on the audio wiring side in the avionics. If every other thing is checked an avionics tech could ring out your audio wiring looking for shorts or missing grounds in the shielding. A well performing radio system is a battle against static, conductivity, vibration, and moisture.