There has always been a loud flapping noise when I crank the heat up all the way in my car, say for defrost. It is almost like a piece of paper flapping in a strong wind, but steady. I have tried but am unable to pinpoint an exact location for the noise, only that it comes from the dash somewhere. Also, my ever so observant girlfriend pointed out to me that there is a much smaller amount of hot air output from the two center vents on the car when compared to the two side vents. I did not think this was normal. Any ideas on either of these problems? Are they at all related?

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headrivenoise... listen.

When the heater is on, the center vents are not suppose to let air out. If you have your owner's manual, the air flow is described there.
Females are great in pointing out details. Specially the negative ones...

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J.H.
'86 300E

It's probably the vacuum actuators that control the flaps for the HVAC system. Best way to figure out if they are bad is to pull the glovebox liner, and find the vacuum distribution box behind it. Attach a vacuum pump to each of the lines in turn and see if it holds vacuum - alternately, see if it makes the noises you're trying to get rid of it. The ones that don't hold vacuum will be busted.