I have a 2012 mustang 3.7 V6 manual with ~97K miles and no mods. The other day I parked my car in the garage after a ~30 min drive and turned off the ignition. I began to hear a rhythmic thumping from the vicinity of the fuel tank that started loud and then became increasingly faint, then I heard a what sounded to be a high pitch release of gas through the system. I have heard it twice in the past couple of weeks. I found a thread from "SB 2013 v6" and Youtube video from "amitrius"  describing the exact same issue with a 2013 3.7 v6 mustang...Links below. I have no codes and the car seems to run OK at the moment. I changed the spark plugs and intake manifold gasket recently... Any ideas?

https://www.youtube.com/watch?v=GcDEypwNppA&feature=youtu.be

https://www.mustangevolution.com/threads/2013-v6-air-release-noise-when-turned-off.398834/