The sound is totally normal. It is the sound of the metal cooling and contracting. Some metal will cool at a slightly different rate depending of there is welds or clamps due to the thickness. The next question you will ask will probably be something along the lines of "why didn't it do this last time I drove " or something along those lines. There answer there lies in the fact that there are a lot of factors involved. There is the outside temp, the temp of the exhaust in relation, the barametric pressure can have a slight effect, also the humidity.
If you recently had an aftermarket exhaust or any work done, just check and see if any clamps are slightly loose. It's not a jeep thing.... It's a very common exhaust thing.
