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Thumping noise

I drive a 2005 Chevy Silverado. I only drive a few miles a day, to and from work. The truck has about 32,000 miles. Recently it has started making a thumping noise, but only randomly. From inside the truck, it sounds like someone is knocking from under my hood while other times there are no unusual noises. I opened my hood while it was making the thumping noise but couldn’t hear it from outside the car. Doesn’t seem to make a difference
what I do - put it in drive, neutral, reverse, park, go fast, slow, stop. Any ideas?

Does changing air settings make it start or stop? It may be a blend door, can you hear it better under the dashboard?

The air settings don’t change it, it comes on sometimes with the air on and sometimes with it off. I’ll check under the dashboard. Is it something serious? Should I take it to a mechanic quickly or can it wait?

Make sure the spare tire is tightened properly. While you’re at it, wiggle the exhaust and gas tank to make sure they are tight. Remember to do this when the parts are cold.

It’s definitely louder under the dashboard although changing the air settings doesn’t seem to make a difference.
What do I look for when wiggling the exhaust? By the gas tank do you mean the gas tank cap? If not, where? And what do I look for?

If it is a blend door it is not an emergency, but would be good to have someone make sure that is what it is.