Case fan making a grinding noise and I found little plastic bits - do I open it up or send it back?

Those black bits are almost certainly the fan's own blades. The wobble plus the dragging sound when you flick it fits perfectly with a squirrel cage impeller that has started shedding chunks. When one of those lets go it throws pieces out the vent and the remaining blades grind against the housing, which is exactly the card-in-the-spokes racket you heard. Nothing from outside got in, the fan just ate itself. Young fans do this sometimes, ussually from a cracked hub or a blade that was weak from the start. Running mostly PLA and keeping the chamber clean means it was never debris.

Same thing happened to mine after about five months, black crumbs all over the shelf under the vent and a horrible grinding noise mid print. Agreed with the diagnosis above, when I pulled mine apart one blade had snapped clean off the hub. I emailed support with a photo and they sent a new fan without much fuss, took about a week to arrive. One thing to know before you open it up: the connector on mine had a little locking tab that's easy to miss, so give it a squeeze before yanking the wire out.

Before you email support, pull the fan out and spin each blade with a fingertip while looking at the hub under a bright light, because when one lets go the others usually have hairline cracks starting at the root too, and a photo of that makes the warranty conversation go a lot smoother. Mine went the same way and the replacement is just a generic sized case fan with the same connector, so even if support drags thier feet you can have a stand-in running the same evening for pocket change. Keep the broken one in the box though, they sometimes ask for it back.

Whatever you do, don't run it again til the replacement shows up, even for a short job. Back on my old machines we'd slap a dab of epoxy on a cracked hub and limp along for weeks, but I wouldn't now, an unbalanced impeller cooks the bearing fast and drags the whole PSU area down with it. One thing I learned the hard way: check the vent grille and the corners of the chamber for stray shrapnel before you print again. I found a chunk wedged in the grille months after mine went, and it had been rattling loose every startup thinking it was somthing worse.

Still figuring my way around this machine myself, only had mine since the spring, but reading this made me go poke my case fan with a finger just now. Spins fine, no wobble, so hopefully I'm not next in line for the 3am racket. One thing I'm unsure about, does the replacement fan sound the same as the original when running? Mine has this faint hum I've gotten used to and I'd rather not swap it and then spend a week second guessing whether the new one is normal or louder.