hmmmm - not sure on that sound but worth checking the screws that are behind the side access panel either end of the dash. Open the door and you will see a blank end on each side you can just pull off. there is 1 screw on each side that goes in to the dash (towards the windscreen) which are well known for coming loose.
Not 100% this is the cause, but another thing to tick off the list when trying to find out the cause