I think old boy has hit the nail on the head, its the exhaust, as it heats up it with probably either move away from something or get closer to it, mine used to have the same problem but only with someone in the back, I found it was one of the rear exhaust hangers had come adrift and so the box was being pulled to one side. And I wouldn't mind some of that balljoint on the rear stuff as It sounds like good sh*t.