If the motor itself has begun to burn up then there is nothing at all that can be done to extend its life as once the coils start burning up and shorting out the motor often has only minutes to hours of life left in it — such is the death knell of every universal motor once they overheat severely. Other things to inspect would be the brushes and commutator bars as well as the bearings to verify that they are all in good condition as well as they are also wear items.
However, if it is a loose impeller as others have suggested, and that is a very real possibility, then you could fix that but you will, as already mentioned, likely discover that it comes lose once again not long after. However, provided that the impeller is in one piece and has not split at the hub then you may well be able to permanently address the problem by drilling a (cotter pin sized) hole straight through the hub of the impeller, through the motor shaft, and out the other side so that you may then pin the impeller into position with a cotter pin as the cotter pin will keep the impeller from slipping on the shaft or otherwise moving out of position. If the impeller has split along the hub, however, then you will need to replace the impeller as sometimes the pressure of the press-to-fit hub and aging plastic does lead to the plastic splitting where it has been pressed onto the shaft and is under significant permanent strain. Given the RPMs involved, you would not want to keep running an impeller with a split hub since the centrifugal forces developed could lead to propagation of the crack and sudden catastrophic failure of the impeller, so replace the impeller if damaged.
Good luck!
