Sounds like a possible rod knock caused by low oil pressure. Does your oil pressure drop to near "0" or the oil light come on when this happens? It may even increase pressure or the light go dim or off when given a little gas. If so you will need to have the engine checked for blocked oil passages, plugged suction tube screen or bad oil pump. Damage to the bearings is very possible also
how many miles on engine? Do you do your own oil changes? If so , going along with what 'torque wrench' said, go to the parts house and buy one of those long (12") skinny flexible LED flashlights (about $15-$20) and drain the oil (into a suitable drain pan of coarse) ...no don't drain the oil from the flashlight I just had you buy , drain it from your expedition! after as much as possible has drained , maybe after 20 minutes or so, take a good slow look around inside the oil panand observe the condition of the metal surfaces that are visible. If the ford design is kind enough to you , you might be able to get a glimpse of the pick-up screen/tube to determine if that is the issue. if not you might have to remove the pan and in that case refer to answer #1 first.Worst case is you end up with a neat skinny flexible flashlight (good for looking into cylinders with also.