1990 Mercedes Benz 300sd noise under my car

Hello JP,
It is very hard to assist you with a definitive answer. The noise your hearing could be almost anything. I think your friend meant the torque converter and not the catalytic converter. At any rate, you need to get dirty. Start by raising the idle. If the idle is too low, it causes increased vibration. Next, go under the car and start inspecting for anything obvious. Check your engine and tranny mounts. Also, shake your exhaust pipes they may be hitting something. I hope this helps. Larry