Most likely a defective photo-sensitive drum in the toner, however depending on the appearance of the print defect, it may be a damaged/worn fuser roller in the printer.

Measure the distance between the blotches - this will give you the circumference of the drum or roller making the mark. A bit of work with Google should tell you the sizes of the various drums and rollers for that particular printer. Unfortunately, the Select 360 uses a Fuji-Xerox engine, so it's not as easy to get info on it as for most Apple laser printers.