About a month ago.....temps rising...time to tune up the a/c. Hooked up my gauge set, and see that I have only a couple of pounds of pressure. I charge up the system, and the thumping vent thing disappeared. Cool...didnt have to take the dash apart. It was as if there was a vacuum issue going on, and charging up the a/c equalized the vacuum.....I dont know...but its gone.

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1990 300TE....all comments are about this car...in case I forget in the actual post