When I start my truck the engine idles up and then drops down(when its warmed up) I know this is normal and it's the IAC that does that. The problem is when it does that I hear a noise that almost sounds like a knock or ratle only does it as the engine kicks up and drops in idle then it's gone. If I shut the truck off and start it right away it doesn't do it.. if it sits for 1/2 hour it will do it. My buddy(a mechanic) says it sounds like one of the main bearings is going in the bottom end. He told me to try starting it with the belt off to elminate any of the belt driven components.

could it really be a main bearing?? the engine has 138,000 KM's on it. It runs great and has lots of power. My buddy had put in my cam,lifters, intake & new timing gears and chain about 5 months ago and he couldn't believe how clean the engine was internally.

If it is the main bearings how big and how expensive of a job is this?? he says if I'm going to change the main bearings I might as well change all the bearings in the bottom end and install a high flow oil pump..

any ideas??
btw it's been doing this for a good 3 or 4 months that I can remember.. just never thought much of it.
thks

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'88 C1500 350 TBI
Belltech 2/4 Drop,
Nitro Drop Shocks,
3.73 Rear gear ratio,
Edelbrock performer TBI intake, performer cam, timing chain,
Hiflow cat with Rhino dual 2 1/2" out the rear.