sounds like you need to lube the motor teeth, only way to do it is to take the whole thing out really. Not too hard, wind the window down half way till you see the 2 bolts holding the windowto the regulator, undo these then the window should come out with a bit of faffing. Place to one side, then undo the rest of the bolts holding the regualtor to the door & remove.
Depending on the make of regualtor you may or may not be able to split the motor & grease the gears.