Mercedes heating/cooling mystery

I have a Mercedes Benz C280 1996. The heating/cooling vents on the front passenger side blow cool/cold air even though I have temp setting to, say 72F, and it’s cold. In fact, they never blow hot/warm air. When I turn on the ignition, I hear a loudish click from the right side of the car, as though a valve is slamming shut. Anyone have any idea what’s going on?