99 Monte Carlo LS

This is a lonnnnnng post so a huge thank you to anyone who actaully reads it and gives me some pointers.

I recently got a 1999 Chevy Monte Carlo LS. It has about 100,000 miles on its 3100 V6. The engine makes a strange knocking noise when it is cold and sometimes when it is not THAT cold (like if it sits for a few hours it will do it). I am not too good at cars (like knowing how to do stuff) but I know a little bit because of what I have searched for on this forum.

It sounds a lot like piston slap when I describe it but I have never heard this in any other GM (Pontiac, Chevy, Olds, etc) that has the same engine I do. I don't think the knocking I am hearing is piston slap though but if this is AT ALL like your own car or other cars you have been around then I guess it is piston slap...

Here's a "timeline" of any given day:

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When started between one and two hours of sitting idle:

*Piston slap (what I think is piston slap...EVERY 3100 I hear does this) at idle
*Piston slap (a rythmical clacking) at low RPMs under light load

When started between three and four hours of sitting idle:

*Piston slap at idle
*Knocking is not present after startup or during idle
*Don't hear the "normal" piston slap but instead hear a perfectly rhythmical light knock:
Follows engine speed
Heard louder inside cabin than outside of car
Pronouced when revving (or under load i.e. driving), not a back-rattle*Knocking disappears after driving about 50 feet or if the engine is revved to about 2000 for a few second then let back to idle, the knocking is gone and the piston slap noise remains for a little bit.

When stated after over four hours / overnight:

*Rythmic knocking at idle right at startup
Follows engine speed
Heard louder inside cabin than outside of car
Pronouced when revving (or under load i.e. driving), not a back-rattle*"Normal" piston slap is apparent a few seconds after starting car
*Piston slap and knocking exist together
*Knocking goes away before piston slap does

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I will try to obtain an MP3 file once I can find my voice recorder. For now, that is all I can explain.

After reading similar posts and the suggestions you have offered, this is what I noted:

Noise does not change when shorting any cylinder. I have done this each morning over 6 days and it has not affected the noise at all. In my mind, I hope this rules out a bad main bearing on one of the cylinders.

I have ran SeaFoam through the brake booster line in case it was a piece of carbon bouncing around. That had no impact on the noise, either. I did not suspect carbon buildup but I thought I would try it anyway.

I have tried Marvel Mystery Oil in the crankcase just in case it was a sticking lifter. I have read that in some cases a stuck lifter can sound pretty bad and be mistaken for rod knock. I have tried this before two different oil changes and it has not made a difference.

I have changed the oil a few times using different brands and weights each time, and no brand or weight has made any type of impact on the severity or duration. It's exactly the same.

I have already replaced the waterpump and alternator. Both have died.

I have not ran it without the serpentine belt yet (it hasn't been much of an issue until now) but I will this weekend because I am going to change it anyway (it is pretty cracked up). I just ruled out bad accessory bearings in those components but I suppose it could still possibly be in the AC compressor or power steering pump (if it is an accessory that is in fact doing this noise).

Just with the behavior is there anything I should check for specifically while I have that belt off?

I have also had the transmission serviced (new fluid and filter) from an ASE shop I've gone to for years. It does not have any leaks, strange clunks or anything. The transmission does not affect the noise when I shift it into gear or out of gear.

I do not have enough expertise or the equipment to actually pull off the oil pan and check bearing clearances myself but if its something you suggest I will keep it in mind in case the noise gets worse and I bring it somewhere. Thank you for all of you time, have a good day!

Welcome to AF. You can almost guarantee this engine has some degree of piston slap in some cylinders, if this was a lower end noise it would get worse when hot. The looser cylinders will take longer to quiet down in this case. I'd sit tight on this unless a tech listens to this on site and feels this is a " real" concern.

Thanks maxwedge. :)

Just out of curiosity what is that other rapid clacking noise I hear at idle (I know you can't hear it but like I've said I have heard every GM 3100 make it so you may know what I am talking about) when it's started after sitting for at least an hour? Are both noises piston slap?

Valve noise would be 1/2 the speed of piston slap.

I agree that the other noise is lifters/valvetrain. These engines are a zero-lash design, and get a little noisy as they get older. I don't believe either noise you're hearing is anything major serious unless, as Max stated, a tech on-site determines it is more than common piston slap/valve clatter.

Thanks for all of your responses, I was afraid it was piston slap. :(

As maxwedge said, valve noise occurs 1/2 of piston slap.

I get about 6 knocks per second at idle when started (360 in a minute). The clacking (when I hear it after the knocking stops) occurs about 3 times per second (dead on with maxwedge's statement).

How would I figure that against crankshaft rotation speed?

From what I have read people with piston slap problems complain about a light ticking or tapping noise and the clacking I hear is what I thought was piston slap. It is "light" compared to the knock. The knocking (real piston slap) goes away before the clacking but I thought that it should be opposite the valve train should stop making noise when oil hits it and the PS would continue due to having room to expand?

I am just interested in the issue, please don't feel like I am trying to beat a dead horse or expect you all to have an easy fix.

EDIT: The knock definitely comes from the "back" cylinder bank. I haven't singled out exactly which one, but I'm just assuming it's 5 or 6 (whichever is on the back) since those are typically the affected ones GM later coated to reduce noise? The noise is pretty high in the block, probably nothing to be worried about giving way any time soon. The only part that concerns me is that it knocks up there right off start up...could that just mean it's a clearance issue between the skirt and the wall rather than a piston pin to bore type of knock?

Also for future reference have there been any reports of 3.5 or 3.9 slap/knock or has that been worked out? Thanks. :)