We had a communication issue on-board, whereby we were stuck on permanent transmit whenever power was applied to the aircraft.
The fault lay somewhere in the beam or further aft. With this fault anyone who was connected to the aircraft comms were able to transmit externally to anyone who was listening. Disconnecting transmit/receive capability from anyone other than pilot and copilots stations seems to have brought it back to doing what it should. With further checks there's no signal coming out of the VHF unit unless you press the PTT button on the control yoke.
While this may sound like a relatively simple job to carry out. It involved a fairly in depth strip of internal panels to gain access to the various control boxes and cabling that connects the system.