Since you don't have year and model X5 you have in your signature and I don't see where you have mentioned what you have I don't know what vehicle you have.

That said, Upallnight's post is correct. Vacuum decreases as throttle increases. To eliminate a vacuum leak have a smoke test done.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas