There's a knocking noise coming from the back of the engine. It sounds more from the top bellhousing than the bottom of the oil pan. The thing is with a stethescope the noise does not get louder, it just gets faster paced. It's distinctly a knock, but the noise level stays consistent, it does not get louder with RPM.

We tore the bottom end down and the bearings were fine. The rod side clearance was .02" which is pretty close to the .015-.018 standard.

What could it be? The bearings look good, the engine runs fine.

Has anyone ever heard of a loud valve? The odd thing is the noise level stays the same, it's not like a typical valvetrain issue where it gets louder with RPM or changes. The knock sound stays the same amplitude, but increases in frequency with RPM.

 

exhaust leak?
Eric B

 

Cracked flex plate

 

Well this isn't my car, it's a muncie 4 spd. Cracked flywheel, theres no visual damage to the entire clutch assembly? It's an LT1 engine.

It's just one of those things, you can hear the noise, yet with the entire engine apart everything looks perfect. Its not an exhaust leak, as the sounds is consistent, only by the top rear of the intake manifold. The loudness doesn't change, the sound is constant, just the pace increases with RPM. Everything says knock.

The pistons are full floating, could it be a wrist pin problem?

 

Well that does make a difference; the sort of info one needs to know in order to make a sensible guess, let alone an educated one.

What's the oil pressure? Does it change depending on engine oil temp? Any possibility something is hitting the oil pan, perhaps internally ? Any dents in the pan? Is it at crank speed (twice as fast as one cylinder firing), or at cam speed (half as fast as the crank)?

Probably not a piston; those sound more like a rattling than a knock, because they knock twice per crank revolution. Very high rate compared to other engine events.

 

Good point about the piston.

Theres nothing mechanical hitting when we turn the crank by hand, nothing is hitting. Oil pressure is normal, 70psi cold, 30psi warm.

One thing we're going to do is run the engine and use a timing light on each cylinder, see if we can line the knocking rythm up with a certain cylinder. Other than that, theres just no physical/mechanical evidence of a problem, just a noise that sounds distinctly like a knock.