StormZ28

The car: ’99 Z28 A4, stock

I’m trying to track down a noise that I hear under acceleration. Best way to describe it is almost like a header pulsing sort of noise. The type of sound you hear with aftermarket headers that have thinner gauge tubing for the walls than stock manifolds; where you can hear the pulsing of exhaust though the header. Only problem is, I have stock manifolds.

Now, for some more details....

- The noise seems to be a bit louder when the motor is warm.

- The noise is definitely louder when the motor is under load (it’s there under free-revving, but much, much less).

- The noise is NOT present in any way at idle, only at about 1500rpm and above.

- The noise seems to be mostly (or completely) from the passenger side.

- The noise actually seems louder inside the car than outside.

- The noise is not extremely loud or anything, nothing like a bent p-rod or bent valve (I’ve heard those before, and this sound is nothing like that and is much quieter).

- I can’t find any evidence of an exhaust leak at all, I’ve been all over that exhaust system top to bottom & front to back and everything seems perfect. And I don’t think that’s the issue because it’s not the usual “tick” that you’d hear with a leak, it’s more of a “cha-cha-cha” type sound under acceleration.

Some of my ideas:

- I know the AIR check valves can make a fluttering type of noise when they go bad, not sure if that’s the sound I’m hearing or not? I’m not getting any type of SES light or anything. Maybe it’s just the passenger side check valve since the noise seems louder from there?

- I understand that the ’98-’99 cars do have the thinner walled stainless manifolds vs the thicker walled cast iron manifolds of the ’00-’02 years, and that might increase the engine noise a bit over the newer cars. Maybe this extra noise is why GM went back to using the thicker cast iron material for the newer cars, and maybe what I’m hearing is normal?

- Maybe I’m just hearing the typical “sewing machine” sound that many/most LS1s make?

Opinions/Ideas? Keep in mind that I’m pretty **** about this stuff, so to someone else, this noise might not even be loud enough to be a concern.

Should I swap out the passenger side check valve just to see what happens?

Thanks.

intense3123

I don't think it's the check valves. I changed mine recently because they one was making a buzzing sound, like a kazoo. But you can double check by starting the car up, and let it warm for about 1-2 minutes. The AIR pump will turn off. And you also said it happens under load. The pump will also shut off under high load.

JRB 9966

Sounds like a good chance you are missing one or both of the exhaust manifold to AIR pipe gaskets.

Part# 12553617

John

StormZ28

Thanks for the replies guys.

Well, I know for sure that's it's not missing the AIR pipe gaskets. I changed those when I did a recent plug change and had to remove the AIR tubes. And it made the same noise both before and after the new gaskets, so I don't think it's those gaskets.

As for the check valves, the reason I thought that might be it is because I know that these valves sometimes make a "fluttering/ticking" type sound when headers are installed unless you use a restrictor plate. I know I don't have headers, but I thought maybe if the valve was bad it might make the same sound even with stock manifolds.

I checked over the exhaust manifolds and cat pipes for any signs of leaks again last night, and there is nothing. No carbon, nothing is loose.

Again, this noise isn't very loud and for whatever reason it's much more noticeable when the motor is under load, so I'm wondering if maybe it's just normal valvetrain chatter? The car has always done this for as long as I've owned it, so it's not like something that just popped up recently.

Any more ideas guys?

StormZ28

Just checked over the EGR and AIR piping again with an inspection mirror and my finger; can't find any sign of a leak anywhere.

I'm starting to think that this noise is just normal LS1 engine noise and I'm just overly ****.

Any more opinions/ideas?