Coleen, is your machine an antique or vintage machine for certain? There are some newer "Singer" model 15's that that are made in Indonesia or somewhere. The machine-ing of the parts in those machines is no where near the quality of the older ones, and consequently they are very noisy.
If that's not it, then it may be something as simple as bad bobbins. Many come from China are not good. Try to find/use a vintage class 15 bobbin and see if you notice a difference.