I have a 2010 F150 Supercrew 4x4 XLT 5.4L. Sometimes it makes a loud popping noise in the cab on startup. It might pop 3 or 4 times or even up to 8 times. It doesn't do it all the time, of course. Pop is about twice a second. Happens during system check, then no more. What is it? It drives me nuts!