The APU has a signature moan when we fire that up, and of course there are some associated noises when the R and L motors are fired up. Neither of these noises would sound like the noise you described, so I would guess that this could have been the wingfold mechanism. If it had been on display for a while, then of course everything was probably pretty cold relatively speaking (lubrication oils, etc). I've not heard such a noise before, but that doesn't mean that it couldn't happen. Shouldn't have been the VEN's opening up.