Ok it's kind of hard to explain, but I have a 05 f150 fx4 5.4. Recently only when the motor is cold I will give it gas from about 1000 rpm and get a loud pop. Is sounds like its almost coming from under the drivers seat. After that is fine. I could almost make it do it on command now. But once it's warmed up doesnt seem to do it. Can anyone help me please. I want to fix this. I was thinking possible backfire?? Spark plugs maybe??