On further investigation I think this is not noise by grain aliasing - apparently caused paradoxically by the fact the scanner's optics are so good.

Vuescan grain reduction seems to reduce this quite a bit.

: : Simon, It sounds like you have a real problem. I've heard of similar troubles before. A power line filter might help, though I wouldn't expect it to make a big difference.

: I will mail Canon tech support now

: : You should probably return it while you can. Thanks for letting me know that Canon scanners aren't the only ones with problems.

: The Nikon IV ED arrived pretty much DOA.