1999 Dodge Ram rattiling noise

My 1999 Dodge Ram sport started making a rattiling noise.It is light when idle but upon acceleration it is obvious. While driving it seams to shift fine.When climbing a incline it seems to labor and i noticed my RPMS go from 1500-2100.If i ease off the gas a little it will go back to normal RPM for whatever speed i am at. I ran it low on gas a few times and ran out once. The transmission filter and fluid have been changed.It shifts fine on low speeds but at higher speeds 60-70mph is when it happens.What is it?? transmission,Catylac Convertor??