There are some times that there is too much clearance in the thrust brgs for the vanos and there is a kit available to fix it, I haven't had this happen (I don't think) and don't know how similar the sound would be but you could maybe find an example on you-tube. If it's only for a second or 2 I wouldn't worry about it much other than it might be nice just to know what it is.

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