I also have a 2008. The metal on metal "bang" I think you are describing I believe is a service bulletin. It happened to me in the service area at the dealership in front of my service advisor. They updated the software on the vehicle and it never happened again.

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Current BMW's:
2013 E70 X5 xDrive35i Premium (mine)
2014 F25 X3 xDrive28i (hers)
Past BMW's:
2001 E46 325i
2001 E46 330ci
2003 E39 525i
2006 E53 X5 3.0i
2008 E70 X5 3.0i
2014 E84 X1