I have a stock Surly Straggler which comes with Avid BB-7 Road mechanical disc brakes. When I shift my weight to the left on the bike (left crank down, all weight on left foot), the rear brake makes a noise s...s...s...s. When I shift my weight to the right, the noise disappears. So, when it makes a s...s...s...s does that mean that the rotor isn't true? When I shift my weight, does that somehow tilt the wheel a bit?
Setting up fixed pad BB-7s take some perfection for zero noise. For good brake feel you want the fixed pad to be very close to the rotor. This can result in noise. Is your lever feel good? If it is I would say you are fine.
If your fixed pad (inboard) is not close to the rotor you should look into the following. Either your bearing pre-load is loose on your rear hub or your Quick release is loose in the frame.
edit: spoke tension would have no affect on brake rotor drag