There is a rattle in the rear of the vehicle. It is not related to the tonneau cover or the seat latches. It occurs most noticeably at highway speed, and sounds like it comes from the driver side of the vehicle, behind the back seats. Any ideas on what it might be and how to get rid of it?

Check exhaust tail pipe. Grab at the rear with a glove on after vehicle has cooled down and see if you can make it rattle like it is hitting the frame somewhere.