Okay so I have a 2001 jeep cherokee sport 4x4 and when the engine is cold it makes a knocking noise but goes away immediately after I press the gas when in idle and it doesn't make the noise at all when driving. It goes away at idle usually when it gets to 200f but sometimes it pops up here and there but it's especially loud when the jeep has been sitting for a few hours and cooled down. I also has this rumbling in near the trunk when idle and it sometimes goes away, I was thinking maybe something with the exhaust?

 

Evap solenoid might be tapping on the ac line below it. Bend it back up.
could be loose flex plate bolts but that is usually louder.
Get under and look for the rear noise.