But you said you use a short jumper cable normally. So it seems to be pointing more to the TRX, although you don't hear the problem with the 641/TRX.
I'd say the the 'not-solid-lock' of XLRs is the issue. Even though XLRs lock, if there is any sort of jostling, the pins can get jostled and cause noise. I think Documentary Sound Guy has it. The connection probably needs to be secured better. How do you mount the TRX to the boom?