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.