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Yesterday I started my 06 Envoy and as soon as it started a thumping sound was heard from above the rear passenger tire between the panel. It lasts for about 5 minutes and then stops until you have to turn the car back on again. Sounds almost like a broken belt of some sort smacking against the panel. But what belt would be back there?
 

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2004 chevy trailblazer_ls_ext
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It's most likely your rear blend door actuator. I'm guessing you have an extended wheelbase vehicle with rear heat and air? The actuators do go bad, and then they try to home themselves with a resulting thumping noise. They're not too hard to change, just make sure you go easy when removing them. I managed to break the shaft on the rear blend door, and that really made the job a PIA!
Here's a video of how it's done:
Removing the trim panel:

Replacing the actuator:
 
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