>>1627433'Lowest' as in biggest cogs? Could be the derailleur jockey wheel is interfering with the gears. If so and it started happening out of nowhere something could be bent, but otherwise you can adjust the clearance with the screw on the derailleur that sits against the hanger. Put it in the largest cog and adjust until there's a few mm of clearance (smaller gap gives better shifting, but grinding is bad)