The thing to remember with manual gearboxes is that the gears (apart from reverse) are always in mesh. The input shaft from the gearbox has one integral gear, so when the clutch is engaged this turns at engine speed. When standing still out of gear the input shaft turns at something below 900rpm. Press the clutch down, and drag from the input shaft spigot will still rotate it slowly - (a few rpms). This input gear is in constant mesh with the laygear. The laygear is a single casting with a number of gears machined into it. Conventionally (4 speed) that would be input gear/3rd/2nd/1st/reverse. Where you have a five speed box with an 'overdrive' top I would expect a fifth ratio machined into the laygear. Direct 4th gear doesn't need a gear since it works by locking the input shaft to the output shaft. If 5th is direct then 4th/5th would be the other way round. The laygear turns as one unit, and the individual gearwheels sit on the output (third motion) shaft and freewheel at different speeds when not selected.They are helical (except reverse usually) so thrust backwards and forwards on the third motion shaft as torque is applied either way. Clearly a selected gear will deliver much more thrust than a freewheeler, but this has the potential to rattle. Between two gears (e.g. first & second) is the selector hub & baulk rings. This moves back for first, and forward for second gear in a conventional set up. Within the selector rods is a locking plate to prevent two gears being selected at the same time. Reverse is operated by inserting a gear between the laygear and the mainshaft(third motion) reverse gear which causes the third motion shaft to reverse direction. This action causes the crunch if rushed. Otherwise - crunches when changing gear are from the selector dogs which are castellated, not the gears which remain in constant mesh (apart from reverse).
So when decellerating in second with the clutch depressed, the third motion shaft(mainshaft) is being rotated by the wheels. This is rotating the laygear because second gear is locked to the mainshaft. The laygear is rotating the input shaft, and the uneven torque in slowing down is causing the gears to shunt up & down the third motion shaft. So this will whirr, and rattle.
This is a 4 Nerd post....
