My plastic fan blade got chewed up, but is mostly intact. I'll probably replace it though. The odd noise sounded like metallic knocking, almost like valve lifters or vacuum pump that didn't have oil supply yet. It was cold out, about 25F, and the noise stopped after about 2 seconds and everything sounded normal after that, so I didn't think anything was seriously wrong. I am quite familiar with the serp belt tensioner failures, and mine was not failing - good bearing, not cocked, not rough, not leaking, 23kmi old, even had a new spring (I heard of some breaking so I replaced it as preventive maintenance).

Carrameow, thanks for the offer! I'm undecided if I want a used one or if I should get a new one. Was your new cover any different in design, perhaps more reinforcing ribs around the tensioner hole? I'd love to see a photo of old vs the latest version. I'll also post the part number cast into my broken timing cover, I don't recall the full 10 digits offhand. Side question - did you remove the head and oil pan to replace the timing cover? Or did you just slide it in & out with those in place?

Someone else asked about a failed component causing this, or my larger alternator. The answer is 'no' on both counts. All my driven accessories are still rolling smoothly, and if they did lock up, they'd just burn up the belt. And the larger alternator is *less* load than the AC compressor, which was definitely not in use at 25F temps... I think it was just a casting failure, and/or perhaps was overtorqued in the past. (?)