Water under dash.

Ours is a 1.6 petrol Cmax LX manual 2007

We have been plagued with the sound of water gurgling or sloshing to one side on a sharp corner. There is no sign of water ingress in the interior of the car. The car is perfectly fine otherwise with no running problems. The car is well serviced. The engine water level is fine and does not vary. Just had the aircon serviced and pronounced all good. I have checked the aircon condensate drain and it is clear. It is not water in the doors. There is no water visible under the bonnet or the windscreen scuttle.
There is definitely water lodging somewhere and the sound can be heard clearly as though it is just under the dash.

Someone somewhere must had had this problem sorted successfully. Please help if you have any information as to a solution.