This may be completely off beam, and I can see no reason for the directional implications, but here goes.
You mentioned that it was there all day, but then wasn't on a test. What changed, in terms of the radio equipment in use, By that, was it a case of 2 people, headsets etc in flight, and only one when testing? I'm thinking around things like headset connectors in the panel for the second person position, maybe an intercom related issue, are you using active noise cancelling headsets, could a socket connection be vibrating with engine speed, or even (perish the thought) is there an issue with the passenger seat bonding that's not the same empty or loaded.
There is also the implication that it's not just radios, there is also possibly an intercom unit/switching unit somwhere in the loop. Has that been eliminated from the possible suspects. Also the wiring to and from headset sockets to the intercom and on to the radios, any chance of a change here, especially related to ground loops in coax and the like, on microphone leads etc.
PTT wiring, any possibility of a chafe on them that is giving airframe contact at certain harmonic frequencies?
Appreciate that this is very much on the crazy end of speculative, but from what I can see, all the obvious and some of the very much not obvious have already been done to death.
Are there any engine instruments or other instruments which can't be switched off that have a voltage regulator as part of their construction.
Are there any maintenance or other access panels that were removed around the time of the start of this, not necessarily forward of the cabin?
what beacons and or strobes are there on the aircraft, and have they been eliminated from the possibilities?
Is there any possibility of harmonic resonance in something like a control cable run causing contact with an ajacent fuselage member?
I'm about out of ideas at this point, best of luck.