It definitely could be something rubbing and the resonance is transferred into the cabin. I'm not sure its directly behind the Nav, the sound seems to move.
As I said before, the sound is gone since it got so cold (-1 F)
2016 F82 M3: SO/SO • DCT • <Waiting for allocation>
2011 E92 M3: AW/BLK • DCT Europäische Lieferung am 6. August 2010