I have a 1989 Seneca V. The windshield anti icing plate makes a whining noise above 130 knots IAS. I know this is the case because it goes away if you take the plate off. Has anyone else had this problem?
This is a 'late response', however, the windshield de-ice plate should not make a noise...mine does not. If your Seneca is a 1989 model, it is a III, and if it is a 1999 Seneca V, it really doesn't matter, as they both have the same plate.
I would suggest you check the electrical wire going to the plate., If the wrap around the wiring is frayed, you will get a noise, probably a whistling sound!
Good luck.
