I still need to stick my head under the hood when starting. It is not a belt or at least doesn't sound like one. Nor does it sound mechnical like VANOS. Vacuum is possible or the closed loop deal. I'll try to remember to check when I get home.

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'07 650i Coupe - Black Sapphire Metallic
M6 bumpers, skirts & wheels
BMW M Cross Spoke Composite 101 Rims 20"
HUD, Comfort Access