Along with the coupler/jar bearing check try isolating the source: if your model allows, briefly run the base without the jar - grinding with no jar points to the motor fan or base bearings; quiet base means the jar assembly is bad. With it unplugged, spin the blade by hand; any gritty feel, wobble, or up/down play confirms a failed jar bearing. Also check for black rubber dust or rounded teeth on the drive socket, which signals a slipping coupler and is usually an easy, cheap fix.
Yeah that tracks - Good call on heat and fit - most edge cracks come from polycarbonate shells getting brittle. If you’re gaming or fast-charging, avoid doing both with the case on, and try not to remove and reinstall the case often since that creates stress at the corners and cutouts. For your budget, look for a one-piece TPU or a hybrid with a soft TPU bumper and flexible corners; those handle thermal cycles and pocket flex far better than rigid snap-on backs.
