[CobraV8]

In a transmission for a standard rwd setup, the input shaft is locked to the output shaft when 4th gear is engaged. Because of this dyno runs are made with the transmission in "straight through" gear, when there is no load on any of the gears and therefore no gear meshing losses.

This also means that 4th is the only gear with no load on the countershaft cluster gear.

Maybe there's a problem with the countershaft or its bearings? Anyway it's hard to guess what causes the vibrations without disassembling the transmission.

Simon