that is a small leak in your evap emissions system, and it could be many things.
The gas cap is the easiest to check and replace.
Thier is a rubber seal on the gas cap and if its dried out it might be the problem.
Also check the hose clamps down by the gas tank as they can become loose and leak air.
But here is a better discription of what it "could" be.
The GAS CAP code