Known fault? Weird sound when off :(

[mynameisplayer]

I thought I had seen this as a Vanos fault on here a while ago, but the returned searches says no. Why does my *just* out of warranty 50i make this sound?! One of my other cars takes ages for the high pressure fuel rail to bleed down - it will make a little squeal hours after it's off. Is this the same? Never heard it before, can't pinpoint it - intake / valvetrain maybe.

[mynameisplayer]

Start and stop engine to see if fuel rail pressure change makes a difference - no. Leave it for an hour over dinner, come back - still same noise. Put it out of the Garage (fear of a fireball engulfing the M3 and Mustang...maybe) for a bit - noise gone. Plug in iCar dongle and start Bimmerlink - no codes. Sounded like it's coming from in the Vee.