Diagnosis
That's almost certainly the sound of the auger motor straining, either because the pellets are bridging (not falling into the auger) or the auger itself is jammed.
How to Fix It
Turn the grill off. Open the hopper and use the included hopper tool (or a long stick) to physically break up any clumped pellets and ensure they're flowing freely down to the auger. Remove any obviously damp or swollen pellets. Often, just stirring the hopper fixes the noise.
If That Didn't Work
The jam might be further down the auger tube. You'll need to empty the hopper completely and then carefully use a flexible auger un-jamming tool (or a long, sturdy wire coat hanger) to gently clear any obstruction in the tube. Never force it while the power is on.
Heads Up
Consistently using cheap, long, or fibrous pellets increases the chance of jams and that awful grinding noise, which can eventually wear out the auger motor.