Aircraft Sway, Perfectly Symmetrical. Shouldn't Sway?

I've created and tested many perfectly symmetrical aircrafts, put them in 0G with a total force vector of 0, and found that they still move. Is this supposed to happen? It happens when my designs include opposing propeller forces that should* cancel out (*: I'm no expert on the physics of a propeller). It happens precisely when two propellers are dragging a block in opposite direction.

A very simple design that demonstrates this is two swivel joints attached at oppsite edges of the starting block to which I attach proppelers that drag in opposite direction (torque nulled out). Is the starting block supposed to be moving at 0G?

PC: i7-4970, 8 GB ram, GTX 960 2 gb
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If you can post pictures of your machines or .bsg files, maybe I can help you analize the balance. I know the instantaneous stress formula of a propeller.