I found out where it's coming from. I got out of work today and popped the hood to check for the pcv valve. It was out of the holder so I put it back in the clip and closed the clip but the rattle noise was still there. I found out that it in the belt system. I will try to explain this the best I can. Lets start off by the belt is underneath the fan and it goes in the direction of the passenger side of the truck. It goes up to the alternator and to the right. It goes down under a small wheel and then the wheel in the ntop right hand corner of the belt drive has a spinning thing on the front of it that spins a few seconds at a time, and when it spins it makes a rattling noise. I can hear it from the inside of the truck. I will get a pic tomorrow at post it in this post.

 

same noise on mine. changed pcv

 

If it is on the drivers side that device is you Air Conditioning compressor and it has a clutch mechanism that will engage and disengage during use by the defrost system and when you are using the A/C to keep cool...there are only two settings where the clutch does not do this and they're the center two settings on the dash heat/vent control. My clutch is making noise when it engages more like a click right now, but if yours is rattling then yours is looser than mine and should be replaced. You may have to go to a mechanic or dealer to get it fixed...it requires special tools to remove...

 

oops post that says same thing on mine? should say changed pcv valve. sorry!

 

Bad A/C clutch?\

edit: saw post above.. sorry