I'll vote for a pulley with loose mounting nut)s)/bolt(s). I've seen idlers, ps, ac, alt, wp, pretty much any/all kinds of pulleys loose and rattling on their shafts, and they almost always make a ringing noise of one tone or another...

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown