Sounds like the issue with one of the air intake hoses having a crack in it. There is another thread somewhere on here about it. I have the same issue, just have not had time to fix it.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!