Re: BMW intercom squeal in heavy rain
I don't have the BMW intercom but, rather, have the Intarride system. When my bride doesn't ride for a while (measure using the calendar), corrosion tends to build-up on the connectors and creates a squeal when she first plugs in. After removing/reinstalling a few times, the squeal goes away.
Water intrusion seems to be your problem, as the others have mentioned.
Avatar credit: Curtis Callaway, 2009
Rider of Stahlross
1999 Basalt Grey LT
2001 Classic Red Ducati 748