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Where are you measuring the voltage?
Your car has around 25 control modules all of which communicate with each other over the Bus system so with out diagnostics very hard to tell. though you could pull fuses until you find the culprit. But not necessarily the cause
When the new battery was fitted was it fitted correctly? is it the correct battery and type for the car? was it correctly registered to the chassis ?
If the battery is not correct, does your car gave stop/start, then it quite possibly is dead, if it isnt registered to the chassis it wont charge correctly.
Hi battery is 3 months old just went to start car, battery flat even though it started and ran 15 minutes ago. I put multi meter on with key out of the ignition voltage was jumping all over the place, what would make the voltage unstable?
Welcome to the Forum
First how old is your battery ? What is it's condition ? If your getting a "whirring noise" you need to see what it is that's running Cooling Fan ? Recirculation Fan ? Emissions Fan ? Fuel Pump ? Is the key in the ignition (it shouldn't be) If you have a system that is damaged and running try pulling the fuse obviously what ever it is it needs repair/replacement
If Battery voltage is bellow 12 volts at rest "engine off" it is probably dead and needs replacing, when charging you should be connecting the charge at the charge points under the
Try talking to some of the BMW specialists I dont know where you are but my Brother Inlaw has used a guy in West London (Hayes/Uxbridge) on several BMW's BW Tuning I think. Have a Google and talk to them is what I would do