I just started having the exact same issue
Very small vertical moving "noise" lines at various places on the screen of my 27" Mid 2010 iMac. Just like all other reports here and elsewhere the noise or flickering in the thin vertical lines seems to move upwards.
They typically appear as 2 or 3 lines next to each other with a very even distance between them, nearly exactly 1 cm. I've got a cluster of 3 on the left side of my screen, a few clusters of two in the middle and a few on the right side as well.
From my usual working distance of approximately 65 cm they are basically invisible, but from a somewhat closer distance of 40 cm they are very obvious.
It's remarkable perhaps that they aren't visible at all on a completely white background (not even when looking from 5 cm distance), but they are very visible on blue or green backgrounds, as well as on photos. They are somewhat visible on medium light gray backgrounds, e.g they are visible in the gray top window frame of OS X windows like Safari.
Rebooting the computer or letting it cool down doesn't make a difference. I haven't done any repairs to the iMac ever and never opened the case or anything.
The only small unusual thing that I can think of is that a week or so back I got some garbage in my web browser (I displayed one of my own pages in a completely wrong encoding), and when this garbage was on my screen occupying roughly the left half of the 27" screen, I heard a soft high pitched sound. As soon as I covered the window with another one the sound stopped. I'm not sure if this is in anyway related, but just mentioning it.
Has anyone got any success of getting this issue diagnosed? Any clue to what may be causing this?