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05 Trailblazer. Loud knocking sound under dash

(2005 Chevrolet Trailblazer)
in Westerville, OH on February 01, 2013
When Heat turned on in floor or defrost mode, loud knocking from under center of dash. Vent mode is quiet. When I press the re-circ button, it flashes for a second or two, but never turns on... Would this be an accuator? what would I be looking at to fix or have fixed? THANKS!!!
Displaying 1 answers & 1 comments
Popular Answer
on May 15, 2013
Don't know if you ever got an answer but in having the same issue. Going to be taking the entire dash out this weekend. If you know what that knocking is, please let me know. If not, I'll have an answer for you next week.

Thanks,
Trey
on May 15, 2013
It was a blend door actuator of some type. I had it replaced and all is good. The mechanic was able to do it without removing the dash which saved $$$
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