Sounds like a possible injector replacement, had the same issue on my old 2010 a couple of times! Same sounds, does it reset when you turn it off then return intermittently?
2013 X6 50i with Luxury seating, HUD, Premium Package, Premium Sound, Cold Weather Package, Space Saver Spare,3 seats, BMW Apps.
Former "10 X6 35i, Space Grey, Cold Weather, Sport Package with 20",Sport Activity Package, Premium, Comfort Seats, Technology Package. (Buyback)
Former '04 X5 4.4i,
Former '03 X5 3.0 (Stolen)
Former '01 X5 3.0 (Lemoned)
Former 1998 Range Rover 4.6HSE
Former 1995 Isuzu Trooper LS 3.0