I just recently purchased an 03 with the 305 cummins. I have noticed a very consistent rattling sound when i am driving or idling. The noise cycles every 30-40 seconds and last about 4-6 seconds. I cant tell if it is coming from under the dash or the engine compartment. It seems to become less noticable as the truck warms up. Does anyone know what this is?????? thanks

 

It's that thing under the hood. What's it called? Damn it! Oh, yeah....a diesel. Shakes, rattles, but boy she rolls!

Sorry, couldn't resist.

 

I found the noise, It is coming from what appears to be some type of vacuum pump or air pump on the passenger side of the engine compartment. It is mounted behind the battery and infront of what appears to be the computer. does any one know what this is?