I'm hearing a similar noise and thought it was the PCV valve on the valve cover that's not sealed any longer.

This is how my X5 sounds:
https://www.youtube.com/watch?v=H_B3WQsiboU

3.0si, 190k miles

Update 1/17:
Took the valve cover off to install below along with a new gasket kit. Alternator-like noise is gone. My idle seems to be a bit rough though. I might throw new coil packs at it. I've shifting into neutral when the idle is too rough to ignore. It doesn't seem to have issues when not idling. FWIW, I have aftermarket vanos solenoids that I installed to get rid of an engine code.

https://www.amazon.com/11127552281-C...s%2C175&sr=8-1

__________________
e70 - purchased 6/2018
e36 - wouldn't pass smog at 190k 2016
e53 - transmission blew at 225k 2012
e30 - sold as it was a little slow for a DD and burning oil at 220k 2011