Just got a 08 F150 5.4 3V and it had developed what almost sounds like a vacuum leak / sucking noise. Noticable at idle, gets louder with revs but levels out at a certain RPM.

Anyone else ever have this issue? No codes, no rough idle, no loss of power either...

 

I'd start out by checking the factory intake ductwork for any leaks leading up to the throttle body. As a first guess, I would say the previous owner might have installed an aftermarket intake and then taken it off prior to selling the truck. Possibly didn't get everything associated with the stock intake system back in place quite correctly......???

 

Good point. I quickly checked the intake, but didnt notice anything obvious. I may try to take it off, and re-install it to make sure its all put back properly.

For what its worth, it seems to be comming from the passenger side of the engine compartment..