Mate, using it with no or faulty aerial can cook it, I found this out from the manual after trying just that! Mins you, my one survived!
Maybe someone has a spare aerial you can try, or someone with another Garmin and you can plug your radio in to their boat, just to prove a point?
I am sure I have been on another club boat with an annoying ringing Garmin unit, other than Star Turn.
Rob