Thanks for the comments and suggestions. I wonder if anyone else has experienced this experienced this problem, or have any good theories on its cause?
Woody - An underdrive pulley is not for this car, as I have thoughts of one day autocrossing it in SCCA stock/street class. I've now got a new (used) stock crank pulley on the way. (I tried texting you for it first!
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Gelbster - On the accessory pulleys, yes, At 100k+ miles, it's time to preventively replace these sorts of things! And a long time ago, I have some new 5202-2RS bearings for those (same specification as dual-row IMS bearing calls for, I hear!) I read that pressing them in/out of the stamped sheetmetal idler/tensioner pulleys may be a pain, but maybe I can figure it out. I noticed at 3000 rpm, the tensioner pulley had a noticeable resonance. Maybe it's related to the wobbly crank pulley.
Water pump seems fine as far as I can tell, but I'll inspect all the pulleys much more closely when I get the serpentine belt off. I have a new serpentine belt sourced for whenever I need it, but haven't noticed a single crack or other damage on the current belt, so haven't been quick to replace it.
When I got the car, there was evidence of a careless previous mechanic's work in this area. A forgotten large pry tool was retrieved from underneath. (Maybe previous mechanic carelessly pried against the crank pulley, and bent it?) A remnant of a nitrile glove was retrieved from under a bolt head, things in the knock sensor area weren't routed quite right. Maybe a previous mechanic thought this noise was engine knock? Previous owner thought the metallic rattling was exhaust noise, but I think I've mostly ruled that out. Maybe this noise is why he sold me the car.
It's been a good, reliable driver for the last 3 or so years! If I can solve this metallic rattling noise, it's going to be awesome/icing on the cake. Kind of excited to find this wobbling crank pulley!
