Power Steering Symptoms - Failure or something else

[stevekat]

My power steering was making a very low level hum for a long time. My sister borrowed the X5 and it came back very loud, and the steering assist impaired. It is the typical power steering sound when fluid is low, but very loud.

My question is whether this is a failure of the pump, or it not getting fluid.

My reservoir was full, and the fluid looked pretty clean. I siphoned the fluid out. Curious if there has been experience with our X5's whereby the filter in the reservoir could get clogged and prevent fluid from being supplied to the system, or if a hose can fail internally and collapse, similarly preventing fluid to reach the pump.