They are called newton rings. There can be more than one per image. Your scanner should've came with a film holder to keep the film from touching the glass. Drum scanners have that same problem. That is why they use an oil to first coat the drum that that thin layer of oil helps reduce the amount of newton rings that show up. If your scanner did not come with film holders, try using two, thin, black foam cores and cutting a hole in each. Then, mount your negative inbetween the two and try rescanning. Trust me, it'll work. Just as long as the film is not touching the glass.