wierd sound

I have a 99 yukon and a couple of months ago it started making a popping sound. When it's warmed up, it sounds like the exhaust in the rear of the truck starts to pop. I'm not positive it's the exhaust but I don't know what else it might be. It's every 5 minutes or so whether it's still running or not as long as it's hot it sounds like the muffler expanding or something. If anybody has any clue what it might be it would be a huge help.

have heard that sound on my denali also i think its just the metal expanding.