Yeah, I was talking about small end or big end bearing. Highly unlikely though. As the other guys say, look for misaligned stuff first. It's really hard to diagnose without hearing. Is it a constant rattle? More like a tapping? or does it sound like a loose bolt type rattle? In the 300e there are bell shaped bulges on the engine pipes. I think they slow the gases before they reach the cat so your car may not have them being a catless (yummy!) year. They rattled in mine at revs so I had new ones welded in and the noise went away.
Just an idea

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Andrew
300e 294,000mi
380sel 185,998mi
380se 309,980mi