I am unsure of what your problem may be. I see no reason for the clutch to make any noise only during deceleration or for your tranny to. A manual transmission also cannot slip so you can rule out that. It may be a bearing in the transmission, the release bearing or something along those lines but usually the only noise I have ever heard from a clutch was while the pedal is being pushed down or while letting up on the pedal. That's strange. If you figure it out I am interested to hear what it is.