Some interesting discussions, but keep in mind that the problem isn't about transmitting and receiving as such, it's when I push the PTT for the radio on box 1, if and only if box 1 is (a) tuned to certain frequencies and (b) the audio panel is set such that I can listen to the output of box 2, I get a load of noise in my ear. Isn't the existence of any substantial sound a bit odd - don't audio panels in aircraft arrange for a small amount of what the mic picks up to be played through the ear piece to make it all sound and feel more natural?