I had a terrible whinning noise coming from my PS pump for a long time. A cheap mechanic told me to replace the PS pump, but that didn't have an effect. Then I took the car to a true BMW mechanic and he pulled the filter out of the bottom of the PS resovoir (tin can looking thing) and now the car has no whinning noise. I don't even have to replace the filter, as he said that the system doesn't really need it as long as you change to PS fluid regularly. (BMW calls for Automatic Transimssion fluid in the PS system)