This sounds like what you'll typically hear when something is wrong with the EVAP system. It's likely that air is being sucked in past the Gas Cap by excessive negative pressure inside the fuel tank. This likely means the EVAP system is not venting properly, and that pressure builds up inside the fuel tank until air is sucked in as a result. You should check your entire EVAP system to make sure all the lines are in good condition and connected properly. Then make sure your Vapor Canister, VSV Solenoids, and even Gasp Cap are also in working order.