Engine oil doesn't "swish". It's too viscous. Water and petrol will. As the fuel tank is designed not to have the fuel surging around, I doubt it is that. Otherwise we would all hear it every time the tank started to run low. There is no surface large enough in the cooling system to let the coolant "swish" so I would check the doors. I had a Cavalier where water collected in the driver's door. It didn't "swish", but when I checked and cleared the drain slots, at a routine check over, about a gallon of water drained out!