Audi s1 bubbles in coolant

[danycee]

Hi guys  I have recently brought a audi s1 2016 72k and noticed my coolant was low when getting home i topped it up and kept my eye on it but noticed their was quite alot of turbulence in the reservoir when the car was running and wasn't sure if it was normal or not so I had a mechanic come out and check he said it was normal flow from the waterpump return. A few days later I noticed a small steady stream of tiny bubbles coming across the seam of the tank so I waited for the car to cool down but still quite hot and took of the cap to inspect the flow their was a nice hiss and the coolant risen up the tank  and i got a friend to rev the engine and their was alot of bubbles and what looks to be froth from the return hose  on acceleration the car was up to temp I don't seem to be losing any coolant the heaters blow boiling hot and consistent throughout the vents I've changed the coolant cap for a belstine one  and used a chemical leak test wich stayed blue is this possible head gasket or possible air in the system or just turbulence. Thanks here Is a video when accelerating