Transmission making loud clanking noise

I have a 2002 WS6 T/A with a 6-speed tranny I had rebuilt by "sixspeedsinc", of course they are out of business now. It is making a loud metal on metal clanking noise when the car is not moving, parking brake on, in neutral gear, but only after letting off the gas. So, going from say 2000 rpms back down to idle. It does not make the noise at idle or while you are giving it gas. Also, the transmission has been leaking fluid from where the tail shaft meets the main section. Could low fluid be the cause?

Well, the leak is fairly common, and easy to change. The output shaft seal is around 10 bucks. The noise could be many things, get the car up nice and level or go have the oil level checked. Usually that seal will just seap and not actually leak, so your probably fine on oil level.