this is tough to diagnose without actually hearing it but: (harmonics are tough to narrow down)
A) just because muffler looks good doesn't nessesarily mean the baffles inside aren't blown out. (you would normally notice that in all RPMS)
B) Could be wheel bearings not diff
C) Could be fan clutch
D) Could be trans
E) Could be center bearing (driveshaft)
When was last full maintence? If you're not sure or lack the tools - have a reliable tech, BMW dealer or local shop that you trust have a look at it.