Just a guess on my part, but it sounds like the bearings are gone or something.
Call Marshall's parts department and order a new fan. (maybe even check to see if they have a website where you can order parts)
And don't keep using the old one in the meantime. It's a slim chance but the extra friction could cause a fire.
You could also order one through a music store but you'll pay extra for it and end up waiting longer.