surfblau,

I am not sure about the 1987 300SDL, but a W123 Diesel will make pretty nasty metallic banging when the air cleaner mounts fail. These are little rubber elements sandwiched between two metal ends with threaded extensions. When they break (the rubber fails) the air cleaner housing bangs around and makes a nasty noise. They are a couple of bucks each and take pretty little effort to replace. I have done the change, and recommend doing them all instead of only the failed ones. Heat and time are what ruin rubber, so if one or two fail, the other one(s) are about to.

These items are the subject of a number of threads, so a search if you need more information should be the quickest way to get data. Hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)