If it hums, there is still electricity going to it. Take the allen wrench and turn it from underneath. If you can't find the allen wrench that goes to it, you can put a socket on a long extension through the hole and grip the center nut on the assembly. When you do get it free, feel or look down in there to find the offending particle. DO NOT turn it on again, as this shard can fly outward and punch a hole in the outer casing. Don't ask me how I know.
Steve