Are the vent hoses kinked? There's 2 ports on the front left of the tank. The forward one relieves tank pressure/routes to evap canister. If you still have the canister installed, the servo valve that routes the evap canister air back to the secondary air ports at the cylinders, could be stuck close.
You can have the bike sitting in the sun and enough pressure will build up to hear the vent, pressure builds up very easily in the tank. It's not fuel expansion but just the air warming
