My Mom gave me her old Singer 513 Artista. It had a broken feed gear. So I bought and replaced all of the gears. I even bought the repair guide so I would do it right. Well, now it has a bit of a buzzing type noise coming from somewhere around the bobbin. The gears look ok, not like they have been trying to eat each other up and appear to run smoothly. Any thoughts as to what I could have missed during my repair or any additional repairs that need doing? Any help is greatly appreciated!