DIAGNOSIS That grinding is almost certainly the plastic oscillation gear wearing out or stripping. It’s a weak point in the design after a lot of use.
FIX You’ll need to replace the oscillation gear kit. 1) Unplug the fan. 2) Lay it down and remove the base (usually a few screws underneath). 3) Carefully separate the outer shell to access the motor assembly. 4) Locate the small white plastic gearbox attached to the motor. 5) Remove the old gear assembly (note how it fits) and install the new one. You can find the specific replacement part online by searching for "Lasko 2511 oscillation gear".
IF THAT DIDN'T WORK The noise might be from a foreign object—like a hair or string—wrapped around the central post the fan turns on. With the fan disassembled, check the post for any debris and clean it off.
HEADS UP When you reassemble, make sure the fan head is seated correctly on the post before tightening everything. If it’s off-center, it can bind and cause noise or stop oscillation entirely.