Had this on my Mrs Ka ever since we bought it years ago, and I've been driving it alot recently and it'e REALLY starting to bug me!
Put it back to the dealers twice for it to be inspected under warranty, but nothing was ever found.
Car seems fine if you drive normally, but if you decided to go round a corner hard, then the PAS will start making a droaning sound, and will keep doing this until you turn the engine off and leave it sitting for a while.

You could drive for miles and go round 1 corner too fast and it'll set it off, then it'll do it all the way there, even if you didn't go round anymore corners.
I'm pretty positive that's what's happening, is there any easy fix for this?? p.s the fluid is at the correct level, and there are no issue at all, car has done this from about 8k miles, and it's now at 36k. Steering seems to work ok even when it makes this noise.

 

ive had similar with various audis, changing the fluid cured it in these instances.