Re-reading the original post, the problem occurs when:
- Engine RPM is 2200RPM or greater
- Tuned station is directly off the nose
- Is a receive only effect
- Noise does not change in pitch or volume (i.e. it is on or off)
- Noise sometimes is not present, other times it is
- Only on VHF
So, the headset has no idea about any of these things, so let's eliminate that.
So what's possible?
1) Above 2200RPM the received signal strength is significantly reduced?
2) Above 2200RPM something is generating a wideband VHF whitenoise signal? But the problem with this as a theory is what does that have to do with the direction of the source (wanted station) - nothing, so this does not make sense.
If the coax was bad (for both radios!) then how does the orientation of the aircraft w.r.t. to the source station matter? Nope don't think that is it.
What does the hand-held tell us? The fact the hand-held was OK, makes me think this is not generated RF (per [2] above), but to me reinforces the idea that the problem is reduced signal strength when the station is directly off the nose, and related to the aircraft antenna installation w.r.t, the engine/prop orientation.
Unfortunately I really can't come up with a believable theory for this right now.
- GY