When you've taken it to your LBS, can the mechanic hear the noise when the bike is in a stand?
Have you checked the hub of the problematic wheel carefully - not the freehub but the rest of it? Wondering if you've got a cracked flange or loose spoke that only gets noisy when there's both weight on the wheel and drive tension on the spokes.
Do you feel any roughness coming through the drivetrain to your feet on the pedals when it's making this noise?