Feel and hear it. I believe it is expanding and contarction of some part of dash, door or floor due to the air pressure changes caused by the speakers.

 

I'll have to check that out the next time I go out. I'll let you know.

 

Is your fender loose? I had the same problem and it ended up been a loose fender hitting the body.

 

Check to see if your battery is loose, really push on it. If it's loose it will slide a bit.

 

Take off one of your swaybar links and see if there is play in your swaybar....

 

sounds stupid but check out sound system.

 

Did you find out what it was?

 

curious here as well, the fact that it goes away when the jeep warms up is the kicker. that is why i mentioned the exhaust, and someone else did also about oilcanning. There are a lot of things it could be, but when you throw in that it goes away when it warms up, that excludes most of them.