this happened to my mp40s
the shaft started not to turn freely, so the metal nut (that sits in the round magnet) that holds the drive shaft eventually melted the teflon washer. after that it was metal on metal as the nut gound the base. eventually the shaft-magent-propeller assembly stopped spinning.
I think it was due to improper re-assembly of the wet side after thorough cleaning. the plastic nut between the propeller and outer housing was mis-tred and then overtightened the shaft.
long story short, I sent it to Ecotech and the sent me a free replacement wetside. I always have a spare anyway, so I wanst down any time.