I know this is an old thread, but for those who arrive here via internet search for a solution to a noisy bobbin area on a 15-90 or 15-91, you may try checking your shuttle driver cushion spring (page 28 of 15-91 adjuster's manual, just google for it). The spring tends to flatten out over time and allow too much space, resulting in the click-click or clack-clack. You can tighten up by bending the spring out a bit; but be careful, if you bend too much you will have issues with thread not getting though and breaking. Or, you can always just replace the spring with a new one, Singer part number 125298. I'm not a professional, this is just what has worked on my machines.