1st off I have the 4.7L so I don't know if this also could happen with the HEMI. When I start the truck after it's been sitting for over a few hours the fan sounds like it is going warp speed (I don't know if its the fan or not but thats what it sounds like) and the RPMs are running high. It usually lasts for 60-90 seconds and I try not to shift into gear until it stops but sometimes I'm in a hurry and it gets really loud and then nothing, quiet as can be!! I asked the dealer and they said it was no big deal but couldn't tell me what it was. Any thoughts?!?!