Flying dog, this sounds to me like RF feedback (as mentioned by Andy@coffs). Some radios and headsets are more susceptible to it than others but the biggest factor is the distance (or lack of it) of the antenna from the headset. As the Jabiru is a GRP aircraft, there won't be the same degree of shielding that you would get with a metal aeroplane. Are the mic and phone jacks located behind your head? If so, this can make it even more susceptible. The wires to the jacks should be shielded and so should the headset cord. I have seen headset cords that were unshielded ... that's asking for it! As the aircraft is not yours, it will be difficult for you to experiment. Firstly, try a known good headset. Secondly, try your headset in another plane. In extreme cases, I have had to shift the antenna to the belly and install it with a sizeable ground plane. This usually fixes the problem in aircraft such as Tiger moths which also don't have much natural shielding. Good luck. John (retired avionics LAME).