My 2001 C240 (36K miles) has developed a loud rattle upon starting, idling and accelerating up to 2000rpm. The noise quiets down at cruising speed but increases during idle and acceleration. The sound seems to be coming from under the front seats / rear of transmission.

The engine is running fine, transmission shifting properly, no noise coming from under the hood.

My first thought was loose exhaust system or a broken exhaust pipe hanger. Checked that out and the exhaust system is tight and and not vibrating.

Any thoughts? Thanks for the wisdom.

Driveshaft? Center support bearing makes these sounds under load if its starting to give up the ghost.

Thanks John -Not familiar with the center support bearing, is that bearing part of the flex disk? in the transmission? Is it an expensive repair?

By "starting", do you mean starting the engine? Car not in motion?
By "idling", do you mean engine running, but car not in motion?

If the noise is present when the car is not in motion, the drive shaft center support bearing is probably not the culprit.

Yessir - Starting = ignition ON , this is when it appears to be the loudest
Idling + engine running / car not moving -

While in motion, pretty loud under 2000rpm, seems to quiet down at speed but still faintly present

If Drive Shaft Center Support bearing isn't the problem, what do you think is? Flex Disk?

Get it on a lift and poke around underneath with the engine running. That's the only way to find the rattle definitely.

__________________
CENSORED due to not family friendly words

Could be just a loose heat shield in best case or a broken up catalytic converter core in worst case. Get under it and bump some stuff around on the exhaust system. I had a little Geo tracker that made a hell of a racket, just as you say. Heat shield had cracked out under the bolt heads and it was rattling around. Friend just had to replace the cats on her Toyota truck because the cores had broken up and were rattling around inside. If it is the cat cores, the car may also lose power and get worse fuel mileage, as the exhaust is restricted.

Sounds like a "rattle"? Check the catalytic converter.

__________________
62 220sb
67 250S
72 280SE 4.5
74 280C
77 300D
82 240D
85 190E 2.3
86 300E RIP 12/28/09
85 300SD
92 300D 2.5
00 E320 Current
Over 1,000,000 miles in Benzes, Since66

....and a whole passel of BMW 2002 and Tii

They get old, don't last the life of the car. Usually it's engine vibration at low rpm that rattles your teeth, such as yours, but it could be other symptoms.

That would. be best - I was poking around underneath to see if it might n=be a loose clamp or hanger on the exhaust system - because thats what it sounds like - but everything seemed tight - not sure where the heat shield is

Engine or transmission mounts, maybe? I'm bringing her in tomorrow - I'll let y'all know what my indie finds

Heat shields, at least on my 300TE wagon, are on top of the exhaust system bolted to between the trans mount and the center bearing bracket and hanging off the bottom of the converter, itself. My upper heat shield was about to pull the head of the bolt through the sheet metal on the trans mount end. I put a big skin washer under the bolt head to spread the load out.

Well, just got the call from my trusted indie - right side catalytic converter is causing it - not the hanger, heat shield or center bearing - around $1000 to swap it out - not great news - the worst part is he's trouble finding one - any one have an clues on where to pick one up?

Pelican shows them available from MB for a W203 for less than $400 per side exchange. Ready to install with hand tools. I can’t justify welding in generic cats.

https://www.pelicanparts.com/cgi-bin/ksearch/pel_search_2016.cgi?description=Catalytic

Sixto
98 E320s sedan and wagon
02 C320 wagon

Thanks - Just saw that Sixto - called my indie and told him. I told him to check it out - or if he wanted I 'd order it