05 Trailblazer. Loud knocking sound under dash on 2005 Chevrolet Trailblazer

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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!!!
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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.

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 $$$