Whenever I start my truck after it has been sitting overnight or at the end of the workday, I hear a thud under the passenger side floor and firewall area. Seems to happen more so during cooler or cold weather. I have inspected the underneath of the truck and under the hood and found no evidence of anything wrong. Anyone else ever come across this noise?

2003 Crew Cab V10 6-Speed 4x4

 

It might be the vent flaps opening. I believe they are opened by vacuum. I might be wrong so hopefully someone else will respond.

 

I'm guessing the engine might start with the vacuum line capped off? See if the noise goes away.

 

Before you shut it off for the night, turn off the heater/AC for a minute. See if it does the same thing after you start in the morning, then turn on the heater. I'll bet it's the vacuum too.

Pap

 

I will try what your suggesting but I should've included that the noise is a loud thud. Kinda sounds like the floor is being struck with a hammer from the underside of the cab. Sometimes I can feel it as well.

 

The blend door slamming shut makes a thud like that, almost positive thats what your hearing. I always turn off the vent, A/C, defrost and etc before shutting the truck off.(force of habit) Try to remember to do that and see if the thud goes away.