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)