I have a 2015 streetbob last night I added Vance and Hines fuel pak 3 shortshots exhaust performance machine fast air intake. I took it out for a ride today for like an he and when I got home I shut bike off and noticed a hissing sound coming from the gas cap I unscrewed it and the pressure came out then repeated and the sound came back any ideas

 

The gas cap is venting as designed. As the temp raises, the gas vapor expands inside the tank and is vented.