Really sounds like you need a closer inspection, or risk being stranded on the water! Since the metal has been there several seasons now, I would do a closer inspection of the bearings, and gears. Need to pinpoint the source and then change the bearings in the lower unit, and any other worn part. If you are good with your hands/tools, I would buy the manual on the lower unit and watch all the You Tube videos and after STUDYING all the tech data - decide if you want to tackle it. Checking/setting tolerances is very important! so, if you don't have the instruments to properly work them, I'd sub (contract) it out!!