Scan lines - dirty optics or simply not enough resolution?

[Flybye]

This is a scanned negative.

In the out of focus areas, you can clearly see scan lines. Do you guys think this is due to the negative or the scanner? Scanner is an HP Scanjet 3970 with 2400 optical. When I scanned the actual picture, I did not get any of these scan lines. I'm thinking it is the scanner since if these lines were present on the negative, then they would have also come out on the print, right?

[Medley]

I think it's the scanner. Try scanning once in portrait mode, a second time in landscape, then overlaying the images and using blend modes to eliminate the scan lines.

- Joe U.

[Flybye]

Yep, it most certainly is the scanner. Changed the orientation and the lines changed to horizontal. Problem is, this scanner uses a funny negative holder which does not allow me to easily scan a different orientation. Could this possible be dust in it or should I not even bother trying to take it apart to clean?