Why my 1984 Nissan PU Z24 manual transmission makes a grinding noise during compression and when it goes fast but not if it goes at a low speed - 30 mph - Can anybody explain?

I've seen this once before. The transmission was run low on lubrication and the driving face of the main input drive gear was gaulded. At low speeds it wasn't as noticable. At higher speeds it would set up a grinding or loud growling sound. It was in all gears so I figured it was either the main input or output gears. It was cheaper to replace the transmission than worry about trying to rebuild this one.