Heat will not completely shut off. Prior to this happening I head a high pitch sound coming from dash, thought it was the fan in the fuse box going out. Sound only lasted about 5 minutes then stopped. Don't know if that's related or not.

Vehicle is a 1999 ML 320 with a build date of June of 1999.

I was wondering if my vehicle has a heater valve, like a monovalve on a 124 which will give similiar symtoms. When searching I read that some early ML's use a blend air system, whatever that is.