Sounds like you might have blown a seal in the rack ?? I take it the power steering pump is quiet even when trying to turn the wheel? Did you have work done on the rack or is this a rebuilt? If you can get to the pressure hose to put some restriction on it the sound of the pump "might" help with diagnoses.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)