After a long drive this weekend (about 500 miles round trip), the sound is completely gone. Haven't been able to reproduce it. Has to be pebbles/rocks, and either they are stuck in something or they dislodged and are gone.
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel