2001 Hyundai Fuel

Hi, My Mother has been having some trouble with her Hyundai Accent for sometime now. When you fill the fuel tank above about half way or more, the car drives very poorly, running very very rough, running a bit smokey and very sluggish almost stalling at the lights needing a fair amount of clutch to get it going. As soon as the fuel is below half way in the tank, there is nothing wrong with it, runs fine. Sometimes she says she can hear a whining noise coming from the tank when she parks it and turns it off after driving. It sounds like a faint air raid siren, eventually stopping after about 10 seconds. No one seems to know what it would be. Any Ideas? Thanks!