Didn't have the exact same issue as you, but got digging a while back to see if i had a parasitic drain (battery was giving me issues after only 2 years), wired up a voltmeter to the battery let the car sleep and it was going to sleep, checked the amp drain that way, and kept on removing fuses to see which one could be causing a drain....turns out for me it was the rear dc socket in the boot...it's disabled now, but there are many posts here about the FSR behind the vents near the air con buttons....sit in the car switch of engine wait for the led on the gear controls to go out....during this time you can here a little fan whizzing...both my led and this humming noise stop after about 15mins....if you can hear that fan spinning for much longer could be the fsr..you will find heaps of posts here about parasitic drain....even aftermarket gps trackers that have gone wrong can cause this drain.....

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N