Well, now I've been told that its the catalytic converter innards, probably the honeycomb material loosening. The indie heard the noise when he gunned the engine, causing greater exhaust gas pressure through the system. Of course. the MB warranty expired a while ago. Taking it to the dealer to see how much holiday spirit they have.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)