Strange knocking sound on cold start (video)

[Djurre-Avalanche]

When i start the engine, a hard knocking sound is appearing.
I am not sure where it's coming from, but when i look underneath the car, it is quite loud.
I think the sound is dissapearing when the engine heats up, but i am not totally sure about that.
However, you can hear the sound will become less louder in the movie.
What the heck is this?

5.3 V8 2003 Avalanche with 77000 km (48000 miles)

http://www.youtube.com/watch?v=bgMHK-iJuwY

[thomsen05]

Could this be the dreaded piston slap?   
here's a link if it will help

http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,18496.0.html

[Djurre-Avalanche]

It could be the piston slap problem, but i am not sure, because i don't have the knowledge.

[kiser02av]

Mine has done it since it was new. The dealer said it is a common problem. It has something to do with the valve train. Can't remember exactly what it was though. They said to just let it go, but bring it back and they would fix it free of charge. 116,xxx miles and never had a problem. I would wait and see what others say, but it sounds exactly like mine. and it goes away after a few minutes. I wouldn't worry about it if there is no check engine light.

[Djurre-Avalanche]

Mine has done it since it was new. The dealer said it is a common problem. It has something to do with the valve train. Can't remember exactly what it was though. They said to just let it go, but bring it back and they would fix it free of charge. 116,xxx miles and never had a problem. I would wait and see what others say, but it sounds exactly like mine. and it goes away after a few minutes. I wouldn't worry about it if there is no check engine light.

But did they fixed yours or...?

[kiser02av]

no i never had it fixed. They said it shouldn't ever cause any problems but if it did to bring it in. It was an extended warranty thing from GM. I think it was good to 100,000 miles but I hit that about a year ago. I doesn't hurt anything, just sounds weird. I always tell people that I have on Duramax cylinder in it since it kinda knocks like a diesel. haha

[Djurre-Avalanche]

I did a search on valve train knocks, and it could be that as well hmm....

Experts with good ears...?

 

[Trey]

Hard to tell, but it is probably piston slap or a single valve tapping.   You can tell it's a single valve or piston, because it happens only once every other rev (about 6 times/second at idle).  This is the right rate for a single tapping valve or a piston slapping on the compression cycle.

If it's piston slap, there is no cure, and GM will not fix it.  They claim it is normal.  Piston slap would occur EVERY time you start the engine at a certain temperature.

If it's a valve, there could be multiple causes.  My tapping valve was caused by a cracked head that was affecting the oil flow to the lifter, so it would bleed down and took a while to fill back up after a cold start.  A bleeding valve would occur only SOMETIMES on cold starts, depending on what crank position the engine stopped at the last time.

[Djurre-Avalanche]

strange this is, this is a 2003 Avalanche, so it can't have piston slap because they solved that problem in the 2003 year?

[dns04av]

Mine makes the exact same noise. It's an 04 with 95K km. I had it in at the dealer for some other work and asked them to listen to it first thing in the morning. They said it was piston slap and not to worry about it as long as it goes away after a few minutes.
They also said if I was so worried about it, they would tear down the engine and replace the pistons....on my dime of course...waranty is up.

[Z66 BUTCH]

One thing to keep in mind.....these engines use a lot of plastic.. The intake is plastic and I think the valve covers are plastic, who knows what else.......so they do not cover up the noise that was always there like the old steel & iron parts did.

[Trey]

strange this is, this is a 2003 Avalanche, so it can't have piston slap because they solved that problem in the 2003 year?

They never fixed it.  It results from a manufacturing process change they made.  A long time ago, they test fit each piston to each cylider to match the best pairs.  Later on, they decided to save money by dropping any old piston into any old cylinder and calling it close enough. The result is some mismatches in tolerance that resulted in the piston banging back and forth in the cylinder on the compression cycle.  Anything you hear about a fix or carbon buildup is just BS they spout to confuse the issue.

[yocum c]

Hi, It almost sounds like a stretched out timing chain slapping the cover. Best way to check is to put your hand on the cover & try to feel it hitting... Or try listening  with a Stethoscope on the timing chain cover. 

[Djurre-Avalanche]

Now, i think i have something else then piston slap.
Tonight i took the Avalanche for a spin, and at cold start there was this metal knocking again.
It was almost gone after a few min.
But when the engine heated up, i can hear the metal sound at low speed, sometimes, not always.
It doesn't sound as a loud knock anymore, but more like knocking/rattling.
I stopped the car and it really sounds like metal, but not from the engine, but somewhere underneath the car, maybe in the front, but i think i hear it louder in the rear, not sure about that, could be the gearbox also??
When in Park, and the engine is in neutral, i can give allot of gas, and the sound gets a little bit worse, but when i give less gas, the sound can dissapear totally, untill i step on the gas again!

Like i said, i am not sure where the sound is coming from, but i think it is not coming from the upper part of the engine.

Maybe something is rattling because of exhaust gasses passing by at some point?

I really have no clue.

Especially because the sound can dissapear in an eye blink, and come back also, but when it's gone, you can idle the engine forever, it won't come back untill you step on the gas again.
Could it be the catalytic converter?
There's no check engine light yet.

Sorry for my crappy english.
I'll hope you guys understand what i'm saying.

[MyBigToy]

They never fixed it.  It results from a manufacturing process change they made.  A long time ago, they test fit each piston to each cylider to match the best pairs.  Later on, they decided to save money by dropping any old piston into any old cylinder and calling it close enough. The result is some mismatches in tolerance that resulted in the piston banging back and forth in the cylinder on the compression cycle.  Anything you hear about a fix or carbon buildup is just BS they spout to confuse the issue.

Piston slap has more to do with the change from a longer piston/skirt to a shorter piston/skirt to try to help fuel economy by reducing friction losses. The shorter skirt on the piston does not hold the piston horizontal in the cylinder bore as well - less surface area and therefore less bearing surface - so the piston tries to rotate sideways in the cylinder bore. That's the explanation that makes the most sense that I've found in my research on piston slap.

[VMAX]

My '03 with 59,000 miles just started making a similar loud knocking/rattling sound a few days ago. I tracked it down to the driver's side cat using a stethoscope. With the engine off, you can try tapping the exhaust/cats with a rubber mallet or plastic hammer. It will be obvious if that's the culprit. Just something to check. Should be covered under the emissions warranty in the U.S., not sure about NL though.

[Trey]

Piston slap has more to do with the change from a longer piston/skirt to a shorter piston/skirt to try to help fuel economy by reducing friction losses. The shorter skirt on the piston does not hold the piston horizontal in the cylinder bore as well - less surface area and therefore less bearing surface - so the piston tries to rotate sideways in the cylinder bore. That's the explanation that makes the most sense that I've found in my research on piston slap.

I think if that were the case, the rings would absorb the small amount of play that results from the piston rotating in the cylinder.  Piston Slap is the result of a knock back and forth, enough that the rings would come out of contact with the wall and bang against it.