I know but there a degrees on moisture and types in the UK.
It can be slashing it down with rain and you won't get any problems but 3 days of fog and you will get all manner of issues with avionics and the like.
The fact there is no problem when its on battery says to me that there is a leak from the earthing side allowing noise from the alt to effect the radio.
This can be for numerous reasons, one of which is that all the connectors are soaked and have condersate on them. A waz round with a can of spray and rerack them and everything will be grand.
Or it could be the back emf thing on the radio DC supply has a water bridge and noise is getting in that way.
If the aircraft is sitting dry in the hangar it would lead me to suspect the radio is fecked internally