Hollow knocking Sound around 70-80mph

[apowerX5]

I just started to hearing this hollow knocking sound last night while driving down the highway. I only hear it when I get around 70-80mph, sometimes its only two knocks other times its a few more knocks. Once I let off the gas it seems to go away. I don't hear any other noises or experience any other problems while driving at lower speeds. All gears seem to shift normal with out any problems. Only other thing that has been going on is rough idle while in D or R at stop lights, etc. Does not idle rough in P or N.

I'm afraid to ask what everyone thinks because this is sounding like a transmission issues...Any Ideas????