It seems that I have the same issue.
I noticed a squel (kind of a feedback) when I transmitting. It's better when engine is off or at low RPM and gets worse at cruise power. However, it varys and it is difficult to track down, but at least very annoying.
I changed sidetone parameter and tried with different volume settings, both at the I/C and at the headsets, but doesn't seem to really help much.
I am using Bose A20 headsets for both seats and a this Inflight solution
to feed I/C audio to my GoPro 5
My Radio is a Garmin GTR225
If somebody has solved this or could provide more pointers to tackle this, that would be great.
Somewhere I read that a ferrite core might help.