Something is..not right.

[Kjz99z28]

Ok.. I am not exactly sure how to describe this, but I'll try.

About a month ago, when we saw a few cold days, I noticed my car (99 Z28 w/ 90K miles on it) would make a strange rattling, metallic noise of some kind when it hit a certain RPM. I can't get it to do it all the time, and it's only when it's cold out, until the engine warms up. It comes from under the hood, and almost sounds like the passenger side. (could be wrong on that though.) It didn't do this last year, and it annoys me because my old car (a 94 3.4L w/ 155K miles) did the same thing. It's loud and kinda embarrassing. Any ideas?

[Mr. Luos]

Piston slap?

What RPM? Could be a little detonation.

[Kjz99z28]

Is piston slap noise very rapid? Because this almost sounds like something is rattling and hitting something else.

It's at low RPMs, usually right after I've stopped and am just giving it gas.

[Mr. Luos]

I can be.

It does go away when warmed?? Lifters might be taking a little time to pump up with cold oil in it.

[Kjz99z28]

Yep, goes away after a while when the engine warms up. I'm just worried because it started with my old v6 when that one started slowly falling apart, and it didn't do it with my 99 last year.

[Mr. Luos]

Maybe try some GM top end cleaner. See if there is some build up on the valves.

[Kjz99z28]

Thanks.

[C&Cbird]

It does sound a bit like piston slap, but hopefully it isn't.

[Wesman]

Sure its not an idler/tensioner pulley going bad?? If the bearing is shot, it can make the rattling/metallic noise that you are describing. See if you can pinpoint where exactly the noise is coming from in the engine.

[Kjz99z28]

I'll try.. it's just really hard because it very rarely will do it while just idling. So usually I'm driving when it happens. I'm fairly sure it is from the passenger side though

[Kjz99z28]

Le bump.

Just cause.

[lvbroham1]

do you have after market headers? If you're still running stockers it could be the gasket between your air tube and stock manifolds. I'd start there, the gasket only cost a couple bucks and it's an easy fix. I had a noise at high RPM and feared the worse, after a lot of trouble shooting, I found out the metal gasket that goes in there was partially gone and metal was hitting metal when I was going pedal to the metal.

[LukesSpeedShop]

I would check all the heat shields just for the heck of it

[basballny2]

my old grand prix gtp did the same thing....it was a 97.....on a real cold day it would make that noise....spring came....went away sold the car 2 my cousin and he hasnt had any problems with the car 10k miles later

['00T/A]

I would check all the heat shields just for the heck of it

this is what i was about to suggest, on my old 2000 silverado as i was pulling out i could pinpoint rpms in which i would hear the rattle, turned out to be a heat shield had it fixed and no more noise ever again

[Kjz99z28]

Thanks guys! I'll look at that.