It might be a good idea to have a second person in the car to help track down the sound when it happens, even better if that person is mechanically inclined. A recording might be helpful as well. As for the purge valve make sure you replace that only with OE.

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