Power steering whine on cold start?

[Alphas]

I noticed that the power steering is producing a whine when cold start, the whine slowly disappears when the engine runs for a few minutes.

 

The power steering reservoir is also having a little foaming, by the way I had the PS fluid flushed, there is no leak anywhere and the workshop has checked but the problem remains. What could be the cause?

[Fuelsaver]

http://www.ehow.com/how_7715778_do-power-steering-whines-cold.html