Probably dust on the print before printing. But could also be little pieces flaking off defective media after printing, although that's much less likely.
Check the area around the roll holder for dust, especially the black, spring loaded things just under and somewhat behind the roll. Wipe with a slightly moist towel.
Dust can settle on the exposed part of the roll facing upwards between uses. I always lean a 32" x 48" piece of foamboard up against the output chute to protect the roll from room dust. Have had almost zero dust problems.
I used to have dust problems big time with the 9880, until I realized I had to clean to plastic cover and the metal plate just under the roll pretty often. That solved all my dust problems and I never once sat there with a blower as the roll ever-so-glacially fed into the printheads. That would be unthinkably, boringly awful and probably never has to be done with any media as long as the whole feed path is clean. The only place you get dust from media is usually right at the edges as it unrolls, and usually only from art papers.
