Tough noise problem in Arrow radios

I'm wondering if anyone here has seen anything similar to this or might
have some ideas on where to look:

A buddy of mine has a new noise problem with the radios in his Arrow,
but it is a little different than the run of the mill noise problem
because it is only in the comm radios (you only hear it during transmit
and when receiving a signal that is strong enough to break squelch,
otherwise his intercom is crystal clear, as are the nav radios, marker
beacon, and ADF). The odd thing is both radios make the same noise,
which would seem to indicate the noise is getting into the system
somewhere other than in the radios. He's had it to two avionics shops,
both of which were happy to take his money but didn't really do anything.

Here are the particulars:

Equipment: KMA-24 Audio panel w/marker beacon, KX155 comm 1, KX170 comm
2, PS Engineering 4 place intercom (PM-1000 I think)

Observations:
We get noise in the audio during transmit and when it is receiving a
signal that breaks squelch (not as strong as in transmit). There is no
noise otherwise.
The same noise is present during transmit and receive regardless of the
radio selected.
The same noise is present regardless of which PTT switch is pressed, and
is audible in both the pilot and copilot positions (didn't try the back
seats)
Noise is gone when not receiving (signal not breaking squelch) or
transmitting, as well as when the selected comm radio is turned off.
ATC reports noisy transmission when we hear the noise, so it apparently
is getting transmitted as well.
The noise sounds like random impulses, but doesn't seem to be directly
related to the engine speed.
When I heard it, the noise was masked by the background noise if I
defeated the squelch. I suspect it was still there, but I really
couldn't tell.
The noise only seems to occur at higher airspeeds. We have not
observed it on the ground or at pattern speeds. (I'm thinking it is more
a vibration issue than an airspeed issue).
Intra-cockpit communication via the intercom is crystal clear, ie. the
noise is only there when you key the mic or the selected radio breaks
squelch.
The noise is also not present if I turn off the comm radio and key the
microphone (I also don't hear the side-tone).
When I listened, the noise level was noticible, but not bad enough to
make the radio unintelligible. My buddy said it gets a lot worse at
times, and has been so bad that when keying the microphone it made him
"want to immediately unplug the mic", and that it made the radio useless.
There may be some correlation to recent rain. He thought the problem
had been solved with static wicks, but then it returned after a heavy rain.
Swapping the KMA24 with another does not change anything, so it is
probably not the audio panel itself.
The noise is not present in the nav audio on either radio. It is also
not present on any of the other audio sources other than comm1 and comm2

My hypothesis is that the noise is coming in through the comm radio
audio amplifiers since it is only present when the radio is producing
audio (either un-squelched receive or transmit side-tone). Since it is
the same for both radios, The source would seem to be from outside of
the radios. If the audio panel had some part that was unique to the
comm radios but common to both receive and transmit it could be there,
but based on what you told me previously it doesn't sound like that is
the case. My thought then was that it was a grounding issue. I didn't
know if the radios get grounded through the cases and tray bonding or
not though.

Things I know of that have been tried so far:
My buddy hadn't noticed that the noise was also in the received signal,
so he was concentrating on the transmit. They had swapped antennas,
tried re-bonding the antennas, and added static wicks without result
before telling me about the problem. They also flew with the alternator
completely removed from the airplane, no change.
The back seat intercom jacks were disconnected at the intercom, no
change as a result.
The radios and audio panel were pulled out and the contacts cleaned.
While they were quite dirty on the kx170, the others were clean. No
change as a result.

We (my A&P and I) took out the trays, cleaned up the mating edges (they
didn't look bad) and bonded the audio panel tray to the airframe with
some braid. After putting it all back together, the noise is still
there but a little different: 1) it is much more apparent in the KX170
than it is in the KX155 now, 2) it is much more apparent on transmit
than it is on receive (not sure if this is any different than it was).
We also played with engine rpm on the ground and got the noise to occur
on the ground, which we hadn't achieved before (hopefully will help with
tracking it down). It seems to be vibration related, and in particular
seems to have some mechanical resonances that depend on engine RPM on
the ground in that it will only start to happen at particular RPMs, but
once it starts it will persist over a wider range of RPM. Once you move
out of that range, it stops and won't start again until you get into a
narrow range (one resonance was at about 1100 RPM, +/- about 25 RPM, but
once started would persist over a range of +/- about 200 rpm).

The audio grounds at the KMA-24 are apparently tied to a bolt on the
back of the tray rather than to pin1 as called out in the KMA24
installation manual, as there are 3 ring lugs attaching 8 or so wires to
the tray. I found this tonight while looking through a copy of the
install manual I found on the internet.