I have a 04 f150 super crew 56,000 mi. It has developed a pop/thud sound in the rear part of the truck. It happens only when you turn left/right. You can bounce the truck up and down no noise. I have removed the spare tire, jack and everything else in rear seat. I have poked,pulled,tugged etc on everything I can get to under the truck ,nothing. Again it only happens when the truck sways side to side. Any ideas ?? thanks