Thanks Davidwozei for replying.
May be a silly idea, could there be trapped water in the drivers side sill? Excuse my ignorance, you'll probably tell me that the sill is solid and no air gap inside. It was just a thought. It sounds as though the fluid is at one end of a pipe, and when then car moves off from a starting position, you hear the fluid rush forward to the other end of the pipe, towards the front of the car. Weird, like water flushing through a pipe. Thanks
Well, when you said Petrol "wooshing", I was thinking a bit too technically...
Here are a few more sounds
A clear "Whoosh", almost crystal clear from the door could be when opening and closing door- that is because locks use vacuum.
On my Right hand drive car I hear the crystal clear "whoosh" when I use Key to open/lock my left door.
If you think there is water in the chassis, it could only be one place. Wait... Did you drive into a river or something?
Well, check next to the jack plugs at the base of the car, there are rubber seals there, check if you can remove and replace then, check if there is any water in there... Though I doubt it. That part of body is supposed to be self draining...