The nostalgia series were 1970s ish, and the serial numbers start with an "N". They're also not potted motor machines (i.e. 15-91) - belted or treadled only. You have a good one. If it's from the bobbin area, I agree, change the bobbin with a known good one. I've had some cheap ones that were too big to even properly fit, and they scrape and pull at the thread. There's also a drop of oil required at the race area, it will be in your manual.
There are also adjustments for the bobbin winder, depending on what your issue is. There's a service manual available in the wild for this machine, on Singer's site even, that would walk you through those adjustments. You can also try tfsr.org, they have a sewing machine manual that will cover it.
I will say that I find the 15s noisier than a number of my machines though. Something about that hook / bobbin area seems louder for some reason.
Also, I assume you've already done this, but make sure the bobbin case "snapped" into the hook area. If it's loose it will make a heck of a racket.