Probably because of the heat and expansion factor.

 

I can only hear the cat rattle at idle but when its moving and there is another rattle that comes from the back and when i put weight in the back it gets louder

 

Your problem is completely different. Obviously something like the exhaust system is hitting some thing like the under side of the bed, gas tank shield, leaf spring, tow unit, or maybe even drive shaft.

 

so i checked it out and my cat has something rattling around inside it, so no worrys.

 

I'm getting the same thing... Now I'm gonna have to go crawling in the morning to check it out. It drives e up the wall. Mine does it all the time, but it sounds like it's coming from right under the engine bay on the driver's side.