DIAGNOSIS Debris is caught in the oscillation gear mechanism at the base of the neck. Hair, dust, and fibers are the usual culprits.
FIX Turn off and unplug the machine. Manually rotate the loop amplifier (the big ring) back and forth while inspecting the base where it connects to the main body. Use a vacuum crevice tool or compressed air to remove any visible debris from the gear track.
IF THAT DIDN'T WORK The internal oscillation motor or gears may be worn or damaged. This is not a user-serviceable part. You will need to contact Dyson support.
HEADS UP Do not spray lubricants like WD-40 into the mechanism. It will attract more dust and can damage plastic components. Only clean with dry methods.