The radio receiver runs an duracells and they are replaced frequently
Also are you sure the boom mic requires it’s battery AND phantom power at the same time???? turn the phantom off and see what happens.
No it has a battery in it – but it functions without. However if we get a re-occurance of the hiss (its being annoyingly fine just now) we’ll try it with phantom on and off and with the mic battery on and off.
Just a thought – you mentioned a possible earth issue with the XLR – is that something that can be tested without getting the camera’s guts out?