I still believe it is speed brakes as yes, we wouldn't be configured then in normal situations.
There is also a very pronounced aerodynamic noise (someone suggested its to do with Dp) around 4000 which is very clearly heard from within the aircraft but this would mean the aircraft is too low.
At / around FL100 the aircraft will be slowing down to 250kts (usually) and thus they may be going from idle back to a higher power setting once this speed is captured?
I would still bet on the speed brakes at that level.