I have a 08 1500 4.7, when I first start it up when it is cold a load rattles starts underneath and last for about 20 to 30 seconds then goes away, I checked the exhaust brackets but found nothing, when I went underneath it seemed to be coming from the transmission, it seems when it warms up or the idle kicks down it goes away and will not do it again till it cools down, have no problems with the ride, does just fine. I was wondering if anyone had this problem or knows what it might be, thank you for any and all help.

 

You have at least one sheared exhaust manifold bolt. VERY common...