Cannot fill Gas Tank

I am unable to fill gas into the tank without gas continually backing up and clicking off the pump. You can put a little less than a gallon in and the pump will click off. With the pump then off you can hear a faint whistling sound as the fuel slowly goes into the tank. After about 10 seconds you can again pump about a gallon in and the pump will click off and the whistling sound will again persist.

Its as though the tank has vent or relerase valve that is plugged. For example, when the gas tank cap is removed air is sucked into the tank. Like negative pressure has been created as gas has been used while driving. I spoke to a chevy serviceman who has not seen this before and instead thought it might be a faulty gas pump. I've been to a number of different gas stations and have consistently experienced the same result.

The local station removed the line that fuel passes through to enter the tank and reported no obstructions.

Any thoughts much appretiated.

Thanks