DIAGNOSIS The drive coupler is worn out or the blade assembly bearings are failing. The coupler is a plastic piece that connects the motor to the blade assembly.
FIX Unplug the unit. Remove the jar. Inspect the black plastic drive coupler in the blender base. If it's cracked or the teeth are rounded off, replace it. You can order the coupler separately from Blendtec.
IF THAT DIDN'T WORK The noise is likely from the blade assembly bearings inside the jar's base. The entire blade assembly must be replaced. You cannot service the bearings themselves.
HEADS UP Never run the motor without a jar attached. This can instantly shatter the drive coupler.