[hunnypuppy]

Okay I found this in a Bimmer manual so likely it could be the fuel pump:

Residual Pressure
Fuel injection systems require a residual pressure to present after the engine is switched
off. This allows the engine to start immediately after the vehicle has been parked.
If the residual fuel pressure diminishes after the vehicle has been shut off. Upon restart,
there will be an extended cranking period before engine start. This is due to the fuel
pump attempting to supply enough fuel for startup.
When the fuel system is at rest, there a three components which allow the fuel system to
retain sufficient residual pressure. These items are, the fuel pump check valve, the fuel
pressure regulator and the fuel injectors.


Page 17: - The Bimmer has a split fuel tank and it's possible the fuel pump was balancing the fuel between the 2 tanks. The noise hasn't returned since:
http://prodcds.bmwuniversity.com/lib...0Diagnosis.pdf

[hunnypuppy]

For those interested in understanding how Power Management, Electronics and Battery Mgmt (for those dreaded IBS, drains and other issues), this is a great read:

http://prodcds.bmwuniversity.com/lib...Management.pdf