a few weeks ago i melted a piston and had to swap the motor in my 99' . i now have a 99 sbrc with EGR delete 07 (whipple sc) 4.8l, 42#inj , 01 flywheel, and a 5spd. I had put an ls2 cam and springs in the motor while it was out but couldn't find a timing cover. drove the truck a couple of times and it had this funny sucking sound for a full 5sec+ after the motor was turned off. didn't have a clue where it was coming from. friday night i put my stock cam back in and it still has the sound. its pulling air through the TB after the motor is turned off. it fires up and runs just fine. i pulled a P0300 code a couple of times but its fixed now. the motor sounds great but it doesn't run the same as it did before. i don't know what this sound is or why its doing it. iv checked everything i can think of. did they make any changes to the newer 4.8s that i should be aware of ? a lot of trucks make the sucking sound for maybe a couple of sec after its turned off but not for 5-7ses. any help would be would be greatly appreciated.

 

I'm guessing that your truck has A/C, and what you're hearing is the A/C system pressures "equalizing".

 

Mine does it, too. It's not the A/C.

 

My truck did that when it had the whipple on it, its just sucking air through the blower to equalize the vacuum in the intake manifold

 

thats what it sounds like to me but it only did it a lil bit with the old motor...

its not the A/C. u can put your finger over the hole in the TB and feel it sucking.

ill stop looking up how to fix it if its no big deal lol