funny noise from heater on 2000 Ford Windstar

Just today I turned on the heater and when the slide bar is all the way right, high heat, it makes a thumping sound. If i slide the bar slightly to the left the noise goes away. Do you know what this might be?

by in Centreville, VA on April 01, 2009
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2 answers
ANSWER by , April 12, 2009
it is a common problem on fords. i had this problem on my 98 taurus and my 99 windstar. the problem/noise is caused by broken nylon sprocket/gear inside blend door actuator. can get at most auto parts stores or dealer. i buy most parts at www.fordpartscenter.net or www.rockauto.com should be able to get for $40 or less. replacement don by removing radio to get into dash area, or from underneath. but through radio usually gives more work room.there are three hex-head screws to remove. actuator sprocket fits back into cooresponding female hole in blend door housing. when familiar with process, only takes a total of about 30 minutes start to finish. if problem was what other commentor mentioned, problem would case hissing. so get yourself a NEW replacement. a used one would probably need to be replaced too soon.
ANSWER by , April 01, 2009
According to my information, your AC/heater controls are vacuum operated. So, do you hear a hissing noise during the time when the controls are at the full heat position? Does the temperature fluctuation of the output air too? If so, you may have a bad vacuum actuator. It could have a leak, letting it hold some vacuum, but not enough to hold the blend door completely open. Some vehicles have electronic actuators and the blend doors break causing a similar condition. Whether or not the temperature of the output air changes is the key to this problem. Good luck.