[muntu]

first cold start of day before actually cranking am getting this fast jackhammer sound coming from the passenger side.seems to be coming from in front of the fan box on the firewall by ac condenser thing.noise only lasts about 20 secs...kinda like a rat tat tat of a pump priming or something. touching stuff in that area i can feel the vibration but it is very light.brand new happening and only happens when cold.nite temps are down in the 50's 60's so cant be glow plug right?

 

[Oneof6]

Try turning your HVAC control to off let the truck run 2-3 minutes and see if it goes away. If it does, and maybe comes back when you put the HVAC in any position then it may be your blend door sticking and that sound you describe is the door jumping. Cost nothing to try it. Good luck, 106.

 

[JakeSpoon]

sounds like the vaccum pump doing its job.

you may have a slow leak developing somewhere.
when you turn on the A/C...does it blow thru the dash immediately or thru the defroster for a while then switch to the dash?

I had the same problem, found fixed the leak for less that 5 bucks.

good luck