On cold starts, the loud noise is probably the secondary injection pump. I just went through 3 of them. The fourth most expensive one on RockAuto finally worked. The first two set codes. The third had different mounting points. The fourth one fit and works great. No noise on cold starts. The pump only works on cold starts. The start up voltage is probably due to the secondary pump running. Once it stops, voltage goes up.

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'06 X5 3.0i - bought @143,123 miles (12/26/20)