1997 Chevy K1500 horrible engine noise

Hi,

I have a 1997 Chevy K1500 that I bought new in '97. It has about 160k miles now. I've changed the oil in it religiously at 3-5k miles, it runs strong and does not smoke or use any oil.

Lately, it has been making a horrible knocking sound when I first start it. It sounds like it's about to throw a rod out the side of the engine. But, after driving about three to four miles, the noise stops completely, and will not return until the truck has sat long enough to cool down again. Any ideas on what would cause this?

Forgot to mention, it's the 5.7L

Does the knock change with engine speed?

The slightly newer LS engine trucks make similar noises either caused by piston slap or broken exhaust manifold bolts (more of a tick but some call it a knock). Both of those noises go away after a few miles. But neither one of those issues are common for the older 5.7 trucks. One possible source of a knock is loose torque converter bolts but that usually doesn't go away when warm.
But believe it or not, there is a GM service bulletin for a main bearing knock. Check it out:
http://ww2.justanswer.com/uploads/he...1500_knock.pdf

The noise does change with engine speed. Also, I drove it some this weekend, and the noise was much quieter, and stopped within about 10 seconds after I started the truck. I really suspected an exhaust leak, but I've checked everything, and it all seems to be tight. I've had several LS powered vehicles, and am familiar with piston slap, and I'm sure that's not what this is. I'm to the point where I think we'll just drive it until it blows up, and then get a new truck. It's lasted 16 years with virtually no troubles, so I guess we got our money's worth out of it :) Only problem is, my wife loves it and doesn't want to get a new one. So maybe just a new engine.

Lifter not pumping up or a crack in an exhaust manifold. Those are my guesses.

Quote:

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Originally Posted by pajeff02

Lifter not pumping up or a crack in an exhaust manifold. Those are my guesses.

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Yeah, those are both plausible, especially with the noise going away. Catastrophic rod knock doesnt suddenly go away. If there's a big enough problem inside the engine to cause such a loud audible knock, then you're likely to see metal in the oil (with the naked eye), or at the very least incredibly high metal counts if you had the oil analyzed by blackstone labs (or similar)