Some of these controls are vacuum controlled at back of the controls...perhaps the "hissing" is a vacuum leak?
Hi Tommy Parry, not totally sure on your problem but would suspect the heater core which is a radiator device inside the passenger compartment. The hot engine water is feed through the fire wall into the heater core when you select heat on the heater. either the heater core is leaking which is an expensive repair or the pipes maybe rubber pipes have a hole in them and checking them in the engine bay especially the hose clamps in the engine bay where the hoses are feed into the passenger compartment may be a starting point.
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