So my 99 4.3 has been knocking cold for 10,000 mile now. Today I started it for 2 minutes till it quit knocking and stopped. Went back 7 hours later and perfectly quiet. Oil pressure 60 cold 40 warm...200,000 miles...Strange

 

Common problem, Mine does this sometimes on really cold mornings, nothing to really worry about, Chalk it up as a 4.3 problem. Dont lose sleep over it
if it does it, and keeps doing it, then worry.

 

is it like a crankcase knock or a misfire knock? my 90 fullsize misfires for the first 10-20 seconds every morning when i start it, sounds like someone is tapping on the cat under the truck but goes away and runs fine.

 

sounds like piston slap...........not from top of engine Runs great 19+ mpg.

 

My did it also but changed the oil to synthetic a few weeks ago and it seems to have gotten a bit better

 

I would be its a lifter.

 

Not sure if it applies here or not but I had this happen on a friends cobalt. It had piston slap real bad when cold. We replaced the camshaft position sensor and it completely went away. Worth looking into though.

 

does the oil gauge come up slowly?

Synthetic oil or add a can of Lucas oil treatment on your next oil change

 

On cold mornings mine has a quiet knock that sounds like a rod bearing for about 30 seconds. Has been doing it for 8 years at least. For the first while I though it was time for a new engine, but it hasn't gotten any worse and I'm somewhat abusive to it.

 

well same here, on real cold morning i start the jimmy up and knocks, im waiting one day, somthing just breaks, but after about 30 seconds or so, sound goes away,
still getting 19 mpg hwy, and 16 around town 189000 miles