Had this on a '93 using MBZ exhaust from the dealer, tried replacing the center muffler with another genuine (local dealer would not warranty) with no improvement. Decided to live with the problem but that car went from whisper quiet to one with a drone.

About the mufflers all being the same/same manufacturer, no. The differences can both be seen and heard.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer