DIAGNOSIS No. The convection fan should have a steady hum. A grinding noise means the fan blade is hitting something or the bearing is failing.
FIX Turn off and unplug the grill. Let it cool completely. Remove the grill grates and the heat deflector shield. Inspect the fan blade at the rear of the cooking chamber. Look for debris like a wood pellet fragment or a melted plastic tag. Remove any obstruction with long-nose pliers.
IF THAT DIDN'T WORK The fan motor assembly itself is faulty. The bearing wears out. You must replace the entire convection fan motor assembly, part number WP2301076.
HEADS UP Never operate the grill with a grinding fan. It can seize, overheat the control board, and create a fire hazard. The fan is critical for temperature regulation.