superb tdi 140

Hi there ...i was wondering if anyone has experienced a noise from the glove box area prior to start up?. Basically, i switch on ignition, but dont actually start and occasionally i hear something resembling a knock coming from the passenger side area.I thought it might be something to do with a flap? as it seems a fairly pronounced sound. It only happenes ocasionally, and all aircon and heating systems seem to be working as it should. I should also point out it only happens now and again.Any ideas as to what it might be? Thanks...andy