Our 1986 245 has developed a strong rattle under very high load. Really, it only happens around 40-45 mph up a fairly steep hill. We have had this car for the last 50,000 miles or so (six years, it currently has 173,000 miles on it). During our ownership it has always had the highest octane gas, and regular oil changes. No leaks of any kind. The rattling doesn't affect drivability at all. It doesn't sap the power. Hasn't affected gas mileage. It stops immediately at slower speeds. Actually sounds like metal is loose, but we can't figure out exactly where it is coming from, and it is hard to simulate the problem on stands :). Does this sound familiar? Thanks.
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86 240 DL/ 94 850 GLT

It sounds like it could be the engine pinging, which could be resolved by adjusting the timing correctly.
The other likely, completely unrelated problem, is a loose exhaust heat shield. These are on the front exhaust pipe (headpipe) and the catalytic converter. Removing them has no ill effects. Usually these will rattle at just one particular engine rpm, and not at other times.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K