The intermittent rattling turned out to be a failed fan clutch bearing. Replaced fan clutch, bearing bracket, and pulley. 2 hrs. tech time.

For good measure, also replaced the timing chain tensioner at less than an hour tech time.

Problem solved; thanks to all for your suggestions.

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ARVY

1999 E430
1999 C230 Kompressor
1992 300E (write-off)
1984 190D 2.2 (sold)