Trouble filling up gas tank

While trying to fill up gas, there's a whistling noise and it shuts off the pump after less than a gallon. It has even shot gas back out of the truck. I replaced the charcoal canister, the valve solenoid, checked all fuses, and still the same symptoms. There is a soft tube that heads up to the filler plate and is capped by a plastic vent cap. The opposite end is just sitting on the frame above the charcoal canister. I am at a total loss.
It seems to be something with not venting enough air while filling or too much pressure in the gas tank. It is a common problem and I've looked at a lot of forums.