I have a Raymarine ST 4000, and the thing has never really worked all that well. Recently, I tried to recalibrate by the owners manual. Unfortunately, the recal display never came up, and now all the thing does (even when NOT engaged) is make this high pitched squeel. I have to shut everything down to reset...which is fine for awhile, then it comes back again and drives me nuts when I am trying to sail. Can anyone shed some light on this phenomenon?