If its making noise, buy a new impeller. Cheap money. They shouldn't be making noise and you should never need to grease them. Water is a lubricant its self and thats all you should need.
Are you taking the impeller apart? sometimes i usally invert the metal or ceramic Shaft and that does the trick. Try taking it apart all the way cleaning all the gunk off it and then cram chap stick into the hole where the rod goes then slide the shaft back into the magnetic part and then put back on the two rubber nuts back on it. once you get it quiet dont fk with it