DIAGNOSIS The combustion fan blade is hitting its housing or a foreign object. A pellet or piece of carbon got past the fire pot screen.
FIX Unplug the grill. Remove the cooking grates, heat baffle, and drip tray. Remove the fire pot. Look directly behind the fire pot opening—you'll see the fan housing. Use a flashlight and a long, thin tool like needle-nose pliers. Carefully reach in and remove any debris lodged between the fan blades and the housing. Spin the fan blade by hand to check for clearance.
IF THAT DIDN'T WORK The fan motor bearings may be failing. The fan assembly is mounted to the back of the grill. You can access it by removing the rear panel. Replace the entire fan motor assembly if it grinds.
HEADS UP Do not operate the grill with a rubbing fan. It can overheat the motor and cause a controller failure.