4spd trans linkage help needed

Pull the side cover and look at the cluster gear and input gear real close, you're looking for chipped teeth.
If the cluster gear has lost it's roller bearings, it will also cause this noise and would also cause chipped gears, due to increasing distance on the thin part of the teeth.

The noise goes away because there is no load on the components, and it's not chipped (or the rollers mangled), too badly.

Mark.