Thanks for (some of!) the replies. I’ve found the root cause of the issue. The simulator has a rocker for PTT and intercom which is spring loaded in both directions; hence the concern about either intermittently transmitting or hitting intercom.
It transpires that this is not representative of the real aircraft and the intercom side of the rocker is not sprung, so I can grip the switch as I have on all my previous Boeings without subjecting the FO to incessant clicking.