ampclamp
Also this:
Firstly a Handheld in the cabin will not pick up the noise at all! You cannot home in on it because it is not there, even with the squelch off.
If you open the squelch on either radio the noise is unchanged, but it is still only there at a certain RPM and when pointed at a tuned station.
If you change to an unused frequency and turn off the squelch there is none of the noise.
The noise does not appear on either NAV1 or NAV2 which are both VOR recievers, it is restricted to the 2 radios. It also does not appear on the ADF audio.
More clues here, which have perhaps been glossed over...
The noise only occurs on an active frequency. Note the effect of squelch off on an unused frequency...
Brain working overtime but not coming up with much
- GY