It's not really your problem to figure out why. First ask (nicely and calmly) about the problem.
What I'd do is scan just a few frames of the material on another scanner, which would isolate the problem to the scanner, as opposed to it being some obscure camera or processing issue. Many digital imaging devices have the facility (not to say requirement) to do calibration on the sensor, but I suspect that's not what's going on here. If this is 16mm, and if (say) the per-pixel black level calibration was off on a any line-array telecine, you'd expect to see vertical streaks, not horizontal.
I can't claim to be an expert on the low-level workings of the Spirit, however, and there are many associated issues that might arise. The thought occurs that any form of vibration or chatter in the transport could create this sort of mess, by affecting the speed of the film as it passed over the sensor. I'm speculating, of course.
I notice that if these are 1:1 pixel representations of the scan, the noise pattern seems to have a period of about 16 pixels, but really that's just idle speculation. Call the people who did it and (nicely) ask.