ATR transceiver interference
 

With our Ikarus C42, with an ATR 500 transceiver, we are experiencing interference. This only manifests itself on "receive" as a crackle and at times could be a comparable sound to an uneven slipstream hitting a michrophone (which is not the case - enclosed cockpit).
On my last flight I switched the Garmin 296 GPS off and the crackle disappeared. I tried switching on/off a few times and, whilst the Garmin was off, the crackle was no more. I even removed the electrical feed lead from the 12v socket to the Garmin and ran it on internal batteries; the symptoms were the same. I hasten to add that this Garmin works perfectly in the owners' aircraft, so itself is not suspect. I have a Pilot 7000 GPS and have experienced the same problem in the same aircraft, but not quite so prominent. Up to this point I thought GPS units only receive but now suspect that that the C42 is picking up some sort of RF from the GPSs. All transmissions sound clear in our headsets and each receiving station give us 5 x 5.
Any ideas?
Richard