Well I've started noticing this huge "thump" sound about 3 seconds after start-up. It feels like someone taking a hammer to the floorboard from underneath, and it doesn't happen every time, but it's getting louder. Any ideas?

 

It's your A/C blend door closing. Still scares me at times. If you turn off your defroster (?) it won't happen.

 

hmmm, I don't think I had my defroster on?!?!?

 

I don't remember if it's the defroster position or not (that's the ?)... But it happens to all of us. Like I said, scarey until you figure out what it is. Try a different position on the heater blend switch. I just try to ignor it now.

 

okay, thanks for the reassurance!

 

i was wondering what that sound was

 

Both of my Superdutys have done that and Ive always wondered what it was, figured it was something with the heater or AC, lol thanks guzzle!

 

Yeah, it still startles me and my wife sometimes.

 

I leave my Heat / AC directional switch set to Vent / Floor a lot, and that noise happens every time I start it back up after it setting cold for a short time. And it still scares the living daylights out of me every time.