The vehicle, a 1999 ML 320 with 92K miles, has a sound which I can best identify as an exhaust leak coming from the engine compartment. The sound is only audible when the vehicle is started, in other words it is cold. Once the vehicle is warmed up the sound is barely audible. It is not valve train noise. For whatever reason it seems to be more audible from the drivers side of the vehicle.

After putting the vehicle on jack stands I started it when it was cold. The sound comes from the driver’s side catalytic converter, I pretty sure about this but by no means positive. The converter done not appear from the exterior to be leaking. The passenger side converter is quiet when started.

Questions:

1- Is the sound most likely coming from the converter as I suspect?

2-If it is not coming from there where might it be from? It does not seem to be coming from the flange where the converter attaches to the exhaust manifold; I say this based on the fact that it did not appear to have any black streaks which could indicate a leak.

3-Is this sound normal?

4- If not normal is it indicative of a converter that needs or will need to be replaced.? (I’ve had no check engine lights)

I Would contact your dealer...Its a Know Problem that the Cats break up ....

I did check with the dealer and they said if that were the case it would not get quieter when it heated up and you would hear the rattle of the broken core inside. They suggested it might be the seal between the exhaust manifold and the pipe (part of the converter) that attaches to the manifold.

I would think I would see a black streak where it was leaking but maybe not.

While I'm not familiar with the ML vehicles, I'm wondering if the sound you're hearing could possibly be the airpump injecting air on a cold start, in front of this cat?

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2007 C 230 Sport.

Manny,

If that's the case shouldn't I hear the same sound on the other side of the vehicle which as another converter?

I don't know if you have air injection into both sides.
Worth investigating.

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2007 C 230 Sport.

I also had a leakiy cat. Seem slike my problem was that the seams were busting out and the air was leaking at the weld seams. Lucky for me emissions warranty covered the problem.

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1997 Silver AMG C36
OEM Advantegarde Grille
OEM Clear Corners
W210 Blinking mirrors
Eibach/Bilstein Combo
Zender Rear Lip Spoiler