Thanks for that Mike, I haven't had much to do with the subtleties of aircraft engines. Then might we suspect something in the airframe resonating with this vibration band which is then causing some sort of mechanical partial 'break' or other disturbance to the 'normal' installation?
What sort of coax cable is normally specified in these installations? Elsewhere it would probably be one of the RG-58 species, but whether that's used in aviation systems or some more exotic teflon covered variety might be expected needs checking. It's not impossible that if the coax were routed through some close fitting areas of the structure that it may in time be subject to abrasion or other factors which could wear through the outer coating and then the shield.
What problems might result from this - assuming that only the outer braid might be touching the airframe both of which can be considered 'grounded' anyway - and why it affects both radios remains something of a mystery.
But a mystery is what we've had all along anyway.
FOR
(thinking aloud before the second cup of coffee kicks in ...)