starting problems

I love my 2006 Fusion, but every once in a while it just will not start, I can hear the fuel pump running, the starter sounds fine, it just will not start. No common links, cold or warm, low on gas or full tank, key start or remote. Yesterday I heard it start to turn over just as my battery was about to try, so I gave it 1more try and it started, but a loud bang came out from under the hood?  I left it running, and then it seemed to be fine.  Sometimes it wont do it for weeks and then sometimes it will give me problems a few days in a row. My son and son inlaw are both mechanics and they are stumped, my son works for a Ford dealership and says hes never heard of any problems