Let's take it apart! After pulling down the silicone surrounding the metal sleeve, it revealed a screw on either side of the unit. After undoing those, the metal sleeve can be removed. Then undo the two upper screws beneath it, one on either side. Then you'll get a picture like below:
There are two screws that hold the motor to the plastic housing, and both of them had vibrated out of their holes and were loose in the cavity! After applying some threadlocker blue and screwing them back in, all seems to be working fine!
For reassembly, it's tricky to get the metal sleeve back on fully. You need to maneuver the impeller while it's inside the sleeve so that it lines up with the hole in the bottom of the sleeve, then the sleeve will fit into the plastic keyed slot near the top correctly.