My vent flap closes with a substantial thump when I put the heater on and later off. Does not sound normal, its as though a damper of some kind is not functioning. The car is a 90 300 SE with 124K. Any ideas?

I have an '86 560 SEL and it too makes a thump or click when the vent controls open or close...been doing it since new. As long as your heat/temp/AC works correctly I wouldn't worry about it.

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1986 560 SEL (159K miles)