The first reply inadvertently took you down a dirt road. An Arai drum is, in fact, a mechanical drum brake, not a disc brake. The drum threads onto the hub body and the backing plate with its reaction arm, actuating mechanism, and shoes is bolted to the hub's rear axle. The drum and backing plate are concentric units that work in the same manner as a drum brake on an automobile, truck, motorcycle, etc... Therefore, the drum and backing plate can, in fact, experience interference issues if the hub body and axle or any of their concentric parts falls out of concentric alignment.

To the OP, what you're describing sounds like what may be a little bit of axle / bearing deflection under high-torque if it is, in fact, coming from the Arai drum. If you're not positive that the drum brake is the source, then something along the lines of what masiman suggets (i.e., removal of the rotor), such as removing the backing plate for a test ride, would allow you to isolate the brake to confirm the noise is not coming from some other part of the rear wheel.

If it is the brake, while I'm not exactly sure what may be rubbing, rest assured that it will reveal itself before too long vis-a-vis some paint removal on the inside of the drum or backing plate, or perhaps around the perimeter. Unfortunately, while I've fiddled around with the Arai drums, I've never had one on our own tandems so I'm at a loss to suggest anything beyond waiting until such time as you're able to determine where the interference is occuring. That is, unless this is a common occurance that someone else on the list has experienced and who will be able to offer some direct and responsive advise.

As always, one of your regional / local Santana dealers (e.g., Rich at gtgtandems.com) may be able to offer some advise.