I recently purchased a 2001 Dodge ram with a 5.9 litre engine w/Automatic tranny, I have noticed a metallic rattle sound coming from the engine while I'm driving.This normally occurs in 3rd & 4th gears and is preceded by a slight resistance accompanied with a hiss sound. I had a 318 in my grand cherokee that did the same thing and I was told it was the valley pan gasket.

Some advice would be GREATLY appreciated

Sounds like to me the bearings in the tranny is shot, it wouldn't be a gasket more of metal to metal contact