There is a belt driving the servo pump?
Is everything OK with it, tight and in place?
Try changing the servo oil. Drain it and put in new high preformence oil
If above fails, then the pump is bad, and you nead a new one

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X5 4,4i-06 Current
330Ci Cab-03 2007-2008
MB E55 AMG-03 2006-2007
Z3 2,8 Roadster-98 2006-2007
X3 2,0d-05 2005-2006
320d-99 1999-2005