The sound thing is interesting. I wonder if it's using an internal combustion engine or something. The one in the studio, when it's hooked up to wires, is super quiet. It's only the ones outside that make that awful sound.
Educated guess? Internal combustion engine. Probably a high end model airplane engine. Its either driving a compressor or hydraulic pump; the joint actuators are too quick and strong for a servomotor that would fit in the space allowed.
I suspect its pneumatic - the tethered one gets its air supply from a remote compressor; ergo the silence.
It is impressive. There's probably multiple 3 axis accelerometers and a gyro or two, all fed into three or four computers.
Guy I bump into at the robotics competitions I attend is letting DARPA pay for his PhD. Hes working on artificial muscles. Scary smart guy.