Crazy Radio Issue, Help Needed.

[MurphyWasRight]

I hope I'm not suggesting something silly, but would it be possible to perform a test with engine running at 2200 RPM with propeller removed? Depending on the noise still being present or not, that could provide some useful clues

A related test has been done, From HF's original post:

I have taken the aircraft to altitude and lean cut the engine, then whilst gliding I have wind-milled the prop with the mags off (and they both ground perfectly) sure enough at 2200rpm the white noise kicks in on the radio.

Since removing the prop and running engine is not feasible anothe test would be to borrow a "seriouse sound system" from a car nut, put it in the cabin and sweep 20 to 80 hz at aboout 120Db spl.

BTW: that level that is basically just above idling for the class of sound system I am thinking of.
I have never seen one in person but the competions for loudest system are intense, blowing out a windshield is a classic beginners mistake I heard. Needless to say nobody is actually -in- the car during the tests.