2003 Chevy Malibu Exhaust Manifold

It's tuff to determine noises without hearing them, but let me ask you a few questions. Does the noise get faster as the engine gets faster?
Does the car run Ok?
Is the check engine light on?
Has the coolant level been stable for the last few days?
And is the cars heat ok?
Get back to me on them and we'll see if we car figure this out.

If I may, can I ask you this, before answering your questions, as they seem to pertain to a coolant issue.

Assuming the noise was emanating from the exhaust manifold (and only there), aren't gases and nothing else the only things that circulate through it?

My mechanic told me the temperature of those circulating gases exceed 600 degrees. And very high temperatures are reached immediately upon starting the vehicle (the engine coolant doesn't reach high temperatures until the vehicle has been running for a while).

Metal expands and contracts based on temperature. Since it is normal for the exhaust manifold to be in contact with such high temperatures, why/how would you get expansion/contraction noises there immediately upon starting the vehicle?

How about if the gases were being partially blocked from circulation, might that be the issue?

As I wrote earlier, the heat shield over the exhaust manifold is intact and appears to be properly positioned. The heat shield is meant to diffuse heat, is that correct? Possibly the heat shield is reaching a high temperature and transmitting that temperature to an adjacent metal and affecting it?

Yes, exhaust gases are the only thing that are suppose to go through the exhaust system. Normal operating temps for the catalytic converter is 1,200 degress F. It reaches this temp. Pretty quickly to convert the emission gases.

The expansion/contraction noises that you inquire about usually happen during the first 3-5 minutes or so due to the different chemical make-ups of the all the different metals on the engine. This will also happen after the engine is shut off due to different cooling times. But as these metals age the expansion/contraction times lessen due to the dissimilar metals being harden and settleing in on their enviroment. Example being a new vehicle will have a lot more of this going on while the metals harden then would an older vehicle.

You stated that you car has approx. 136,000 miles on it, and it's pretty safe to assume that the dissimilar metals have pretty much settled in.
What can occur over time is that the gaskets between these dissimilar metals wears out from the expansion/contraction, which could give you an exhust leak at the manifold, or in intake leak on the intake gasket.

As far as the heatshield go, they are there to protect surrounding componets from the excessive heat, and as with all the other dissimilar metals, they will eventually settle in also.

So if you are gettiing this noise as soon as you start the vehicle and it continues to be there while driving it's safe to assume that it's not an expansion/contraction issue.

Here's a few thoughts. Sometimes when bearings go bad in a component with pulleys, such as the generator or water pump, they can make a clicking/popping noise and will contiune to make this noise from the start. If the exhaust gasket is bad and leaking, it will make a similar noise from the start.

Does the noise change with engine RPM's? Is the first question.

Hope this helps, let me know how you make out.

I don't believe the noise increases when the engine RPM increases.

Two bolts on the manifold heat shield needed tightening. After tightening, it didn't eliminate the noise. My mechanic says that if there was a gasket leak, the check engine light would come on.

Well, like I stated earlier, without hearing the noise myself it's hard to really pin point what's going on. I do have a few more question tho.

What does your machanic say it is?

How long does the noise continue after shutting the car off?

Let me know.

I'm in El Salvador, by the way. My mechanic doesn't know what's causing it. He says that it seems to be located about the manifold and has the sound of thin metal expanding or contracting. He attached a scanner to the vehicle and there were no codes evident. [He successfully replaced the camshaft and valve lifters awhile back, so he knows what he's doing. The exhaust manifold gasket was replaced at that time. The problem existed prior to that repair job, ]

I believe the problem began after the catalytic converter was replaced about a year and a half ago (at the local Chevrolet dealership here; I no longer use them as I don't believe they are competent.). The one they installed was the least expensive one they had (the one they had wanted to install was over $1,000.). It went bad after six months or so. I then removed it and drove it without one.

I recently imported one from the United States thinking a U.S.-Sourced converter might be the solution. The brand is "Eastern". But it didn't eliminate the noise. My mechanic is a friend of mine. He speaks English and Spanish. My Spanish isn't too good, otherwise I'd make more inquiries locally. It goes without saying that if I was in the U.S, the problem would be solved by now.

(I would guess the noise slowly decreases over a period of about 15 minutes or so, until the engine is cooled down - though if it's caused by manifold gases, it 's not a function of a cooled-down-engine.)

Could this issue be related to the quality of the Catalytic Converter that I installed?

I installed a new one recently, the brand is Eastern which I purchased from Rock Auto. http://www.rockauto.com/catalog/catalog.php The product number is 99774. The cost was about $107.00. Don't catalytic converters contain an expensive mineral called platinum or palladium? I don't know how much of these minerals $107.00 buys. Is it possible to buy a more expensive catalytic converter that contains more platinum and that might solve this noise problem?

The other thing that I was thinking was that maybe the heat shield may need to be spaced at a certain distance away from the manifiold - and that maybe it is too close??? That is assuming that the noise is emanating from the heat shield, it being made of a much thinner metal than is the manifold.