Highly unlikely to be anything inside the saildrive. The propeller has a rubber bush between the splines and the metal part and the bond between these two can break down. Little is visible from the outside, but that would be the first thing to cheak. Leave the drive in gear and see if you can turn the blades. The other thing it could be is a loose anode. The screws holding it on to the mounting plate must be tight, or it could be that it is worn and moving around the screws.