Obviously, it could be one of many possibilities, but with that milage on that engine (given reasonable oil change and maintainance schedule) the pulleys/pulley bearings/tension(er) scenario would seem most likely; not engine. Was A/C on when this occurred(compressor)? Did you notice that oil pressure was up? Water pump not leaking/eccentric? Lower end problems are the most rare, so I'd put that at the end of the list, personally. FWIW

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)