I've read this thread several times and the problem is very puzzling. There are no obvious answers - only a large number of improbable single events or combinations which might be the cause. Component substitution - where possible - is one way of isolating the issue, but you've already gone some distance down this track.
On balance it seems highly unlikely that the noise should be coming in through the RF side of the system - i.e. antenna / coax combination. A check on the VSWR for each installation would be worth trying. It's accepted that this checks primarily the transmitting side of any given radio installation, but such a check, if satisfactory, should further eliminate the chance of the problem noise coming in through the antenna. This seems especially unlikely anyway given the expected signal strengths in the regime under test. These should be very strong signals, so much so that their being overloaded by an interference source seems highly improbable. Unless of course it's a large arcing issue somewhere which is more likely to produce a cracking sound rather than a (squelch open) white noise.
It could be useful to connect the hand held antenna input to one of the aircraft antennas if that hasn't been tried (and can be). If the noise is still there and on the HH, then the antenna installation is suspect in some way or the noise is coming in through the antenna system.
If this were to be eliminated, it leaves some other problem common to both radios. Is some sort of critical system voltage level (upper limit) being reached at the 2200 RPM level? Is there any other system indicator which betrays something special about that RPM level.
If it were possible, (and safe naturally) I would like to try the radio/ intercom on a DC supply system independent of the A/C DC rail. Come to think of it, is the main A/C DC system nominal 12 or 24 volt? I'm presuming that the radios and com system are on 12 volt. Can other significant electrical loads like lights etc be switched on (or off) when the noise is present? Does this change things - even in minute levels?
Clues may lie in a critical appraisal of the noise itself. Is it exactly the same intensity / pitch / character as what is heard when the squelch is opened manually?
I'm clutching at straws like everyone else. But these are things which I would follow up if the issue were mine. I hasten to add that I'm not in the aircraft maintenance area. My background in this area comes from being an amateur radio operator with a long experience in mobile as well as other fixed installations. I've occasionally had the need to look into 'simple' A/C radio installations. There are a lot of variables in the way these things are set up, rarely are there any circuit diagrams, or explanations at to why things have been set up as they are, and almost every installation is 'different'.
Congratulations on your perseverence up to this point. There has to be solution somewhere, but for the moment anyway I'm as puzzled as everyone else. I do hope you can get to the bottom of it.
Sorry I can't offer more for the moment. It really is a puzzle.
regards
FOR