PROBLEM SOLVED: pebble in the brake rotor, or so I was told. Definitely a pebble somewhere in the RR brake. Cheers to Seattle BMW for coming through...
2008 E92 M3 6MT, AW/Blk Ext., brushed aluminum
2006 E46 M3 ZCP 6MT Carbon blk/blk(sold)
2001 E46 325Xi 5MT Jet Blk/Blk (sold)