Those look like "Newton's Rings" - an interference pattern. If that is it, then it's the scanning.
Are you laying the negative against the platen glass of the scanner, or are they otherwise touching it (perhaps you do have a holder but the negatives are bowed in the middle and touch the glass)? That is usually what causes it.
If you have a negative career/holder for the scanner try using that. If not, try some cardboard or something at the edges of the negatives to hold them off the glass - you can cut "masks" out of cardboard or some material to do this. You would put the "mask" on the glass, and the negative on top of it.
A homemade mask/carrier may not be a practical solution, but can help determine whether that is in fact the issue.