M3Bliss: I can't find any posts in your link.
The noise is not heard inside the car when the windows are up. Only when down you can hear it coming from under but not sure whether it's the front or the back of the car.
I will replicate it on the way to work and list the exact sequence of events. Thanks for the input so far.
2007 E92 6MT - Silverstone II with /// Performance Exhaust with ACM Test Pipes and BPM Sport Stage II Dev-Tune - Proper engine warm up, 12-monthly oil change, Castrol TWS 10w-60
2014 F15 X5 3.0d