2000 Ford Truck Explorer 2WD 4.0L EFI SOHC 6cyl

My Ford Explorer is leaking gas periodically and no one can't find where the gas is leaking from.The tank and gas line was checked on two occasions with no success. I can smell the gas at times while I am driving because the scent is very strong in the vehicle then smell goes away. No matter what amount of gas I have in the tank the problem still persists.Can you suggest what is the problem?

Thanks.