The problem developed about 2 weeks after I bought my second hand, 2 year old, VRS Octavia. It has been driving me up the wall. It seems the previous owner had the problem previously as there is a selt tapper screw bonding the cylindrical ducting for the heating and the lateral piece with the checker like design on it. If you pinch that with your fingers and wiggle it about the sound is very obviously coming from it.
Its from that exact piece the problem is caused. Im pretty certain if that was removed altogether and replaced with a rubber piece the same shape there would be no creek. In fact there is a lot of plastics throughout the car which dont meet well together. Another is in the central column at around the armrest and hand break. Not near as irritating as the dash creek.
Anyway to get to my point before I started lossening any screws, packing with cardboard and sticking velcro around the place I tried simple WD40 in all areas behind that cover where 2 plastics meet. Its had and instant effect. Now I know eventually it will dry out and probably start again but Im just happy such a simple application has solved it. For now.