I have a 99 E320 W4 with 46K miles. recently I have noticed an exhaust sound coming from under the drivers side and tonite and crawled down there and narrowed it down to the first big device on the exhaust pipe just below the manifold junction. I dont know if that part is the catalytic converter or the muffler. I could not find any cracks in welds to holes in the device. Does anyone know what this is and can it fail internally to make exhaust noise?

Thanks in advance.

Rob