Recently I've noticed a soft thump 3~10 minutes apart...I can feel it as much as hear it...from beneath my truck. It's more noticeable from the passenger's seat and sounds like the vent closing on the AC system. It's annoying more than anything. I crawled under my truck today to pull on stuff to see if anything was loose but everything seems o.k. I hate taking my truck to the dealer so I was hoping that someone here can help.

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'00 5.4L 4X4 Lariat (Black/Silver)
'99 Ranger 4.0L 4X4 XLT

 

I occasionally hear a similar sound on my 99 Supercab. It generally occurs when I accelerate after slowing down, e.g., turning a corner without stopping. I believe it's a vent closing due to some vaccumme problem. I haven't bothered to take it in for repair.

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1999 F150 4X2 SuperCab, 4.6 (Romeo). TuTone White/Gold, Bedliner, Prime Designs Bug Deflector, Ventvisors. No cracks, no Engine Mods, no Problems.

 

Thanks for the response BConehed. Your description sounds exactly like mine. Are there any other vents that could be making a noise other than the one that controls outside air access into the cab?

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'00 5.4L 4X4 Lariat (Black/Silver)
'99 Ranger 4.0L 4X4 XLT