I've been bothered by this rattling sound for over 2 years. Every time I go over a small bump, SMALL not big bump, the back will give me a very annoying metal-to-metal like sound. Tried letting my wife sitting in the trunk and listen, no source found. Searched all over this forum, found some very good tips. Tried but no cure either. So yesterday I said to myself, the hell with it, I'll just try each method again and make it to the extreme.
Stick a THICK felt tape, still rattles; Take the trim & spoiler off, still rattles; disassemble the sliding table, still rattles. Now, adjust the lid stopper ALL THE WAY TO THE TOP, which almost made the trunk lid unable to close. RATTLE DISAPPEARS!!!! It turned out to be I wasn't adjusting the lid stopper high enough to stop the latch from hitting the lower latch when going over a bump.
Problem solved!!!

Just wanna share with you guys. People on this forum are awesome.
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01' 3.0i E53 (M54) with 164,000 miles

07' 3.0si E83 (N52) with 134,000 miles

08' 3.0si E70 (N52) with 91,000 miles