Something electrical still running in boot, when engine off

Z4 E85 2007, 90k on the clock... I have a strange noise from rear of boot... sounds like the tyres are leaking initially but when you listen inside boot or near tyres it sounds like something electric or a fan blowing. The battery is being drained empty, but when I turn ignition on, the noise stops total silence. I've started taking the red lead of the battery, having just purchased a new battery thinking it was the battery... Any ideas what this could be? Do I need to reset the computer after the battery was replaced, is this something I can do with the Key fob ?? either way, the noise continues unless i kill the battery. If I leave it connected, eventually the alarm goes off, I guess telling me the battery is dying? Help?

Do you have a tv module?

As in TV? not in the car No ?

It usually takes around 15 min after the engine is off for all the modules to go to sleep. If you got a sat nav and or cd changer that can make this noise probably as they sit just behind the seats.
Or it might be the roof motor having short. Being stuck. Worth checking.

I had the battery drain problem on mine but I turned out faulty LED light in the boot. It Was draining a lot… Piece of advice I have learned while trying to sort out my parasitic drain- always disconnect Negative cable of the battery first. Never positive as if this touches any metalwork you will have a massive short in the wiring and will probably blow some fuses.

Check if you have plastic 3pin connector next to negative clamp on the battery cable. If YES then this is probably Battery Management Sensor and your new battery has to be coded into the car (this needs OBD connection) to allow the car to properly charge the battery. Apparently if you don’t do this the battery will wear prematurely.

There is no IBS in the E85 so no need to code the battery.
But anyway, the battery won’t be making a noise like that.
Definitely disconnect the black earth lead though.

Is the noise from the left or right side?