well i think it might be my cat but not too sure.. just help me out here because its a little hard for me to explain well when ever i start it up after 10 15 seconds after the revs go down from starting it you can hear a rattling sound comming from the the passenger side in between the cab and the bed. it sounds like if you got sand and put it in a aluminum container and shake it. it also makes that sound when you lightly lightly press the gas you hear it or when your going in reverse very slow with the gas but if you just sleightly give a more gas it goes away both in reverse or in drive. i took it to a mechanic he said it my be my catalytic is clogged but if that was the case wouldnt it be throwing a code out and my truck has no lights going off in the dash. ---------- Post added at 11:58 AM ---------- Previous post was at 11:57 AM ---------- but i think its that big drum of the muffler thats make that noise but not too sure what do you guys think it is?