What makes the knocking sound?

Question

I have just replaced the 350 engine in my 97 suburban (the old one had

150,000 miles with very low compression in two cylinders). I have replaced it with a new GM long block. What is bothering me is a "knocking" sound that I get when it is first started up cold. It is a knocking sound that sounds the loudest around the oil pan. From what I have always been told.. a rod knock gets worse when the engine warms up, a piston slap goes away as the engine warms up. I have the knock for about two minutes and then it seems to be gone. The engine now has 1000 miles on it and the noise has not changed at all. Do I have a serious problem? What is the probable cause?

Thanks MJ

Robert Hancock · Accepted Answer

Sounds like piston slap from your description - in that case, not too muchto worry about..

Nathan Nagel · Answer

Could be an exhaust leak too... I know my Stude has a small leak at theheat riser valve... as soon as the heat riser opens up, it's gone...nateRobert Hancock wrote:

Brent P · Answer

I figure you tracked that down, my mav has a couple small leaks at the manifold gasket, they too seal up once the engine warms.

Nathan Nagel · Answer

I'm not motivated to fix it, I'll be pulling the engine for a cam swapand other goodies this winter when I paint under the hood.nate

bobby · Answer

My 96 chevy pickup has similar symptoms.  Done it since new but much worse incold weather (40ish).  I don't notice it in summer, but during the winter, itlasts for several blocks.  Sounds more like a diesel knock and been told it'spiston slap.  I have 125k miles on it, so I guess it isn't a problem, but isannoying.   Robert Hancock wrote:

Mike Romain · Answer

Older 350 and 5.0 GM engines have a sneaky cold knock that 'almost' sounds like a rod knock.

It is a worn spring on the heat riser valve on one exhaust manifold allowing the counter weight to make the valve knock pretty good in time with the engine firing. Once it heats up, the spring tightens up and no more knock.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

My Name wrote:

My Name · Answer

Thank you all for the replies. I will try to quit worrying about it (keep telling myself that is not serious). A couple of folks told me that it could be exhaust related but, like I said before, the sound is the loudest from the oil pan.

MJ

What makes the knocking sound?

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