Does anyone know the ins and outs of the Vectra heating and vent system? I have noticed, from when i start the car cold, when i first pull away and round the first few corners, a knocking noise coming from inside the vents.

so as i accelerate (not too hard) its like something has rolled back in there and knocked, then round a left corner 'knock' like it has rolled again and bumped into an edge of something.

It goes away after a few mins, just wondering what it could be. was gonna guess that someone had dropped something down there before i owned it but then that'd do it all the time n not just when the cars cold

any clues??