I assume that you are not talking about the regular clicking of the freehub as it rotates, but rather a click during one point in the wheel's rotation. If you are sure it's not anything rubbing on the bike (brake pad, cable,computer magnet, etc.), I would check for a loose spoke (real loose). If that's not it I would recheck & re-grease the hub's bearings- sometimes a single bearing may be sticking. If these are new builds, or were recently serviced, a missing ball bearing can also cause a periodic click (as the other balls fall in line to fill the extra space). Rebuilding the wheel would be a final option.