Icenor, before you try anything else, try this. I use a small magnetic telephone pick-up, available from a high-street electronics shop (Maplin in the UK) for a few $ or £, and attach it with its sucker pad (and a bit of tape) to the outside of one headset earphone. It is plugged straight into the camera socket, which of course disables the camera mic.
The result is near broadcast quality of any intercom or radio transmission in the headset, and of course no ambient engine or aerodynamic noise at all. In the background I hear a faint clatter from the magnetos, but that's all (and in fact is quite reassuring!).
As a solution it is cheap, cheerful and effective, and when I suggested it in another forum some time ago at least one person came back with a huge thumbs up. Do let me know if it works for you.