FOC Question
The FOC conversion converts the transmission to direct drive. Therefore, if the truck is not running and you roll it backwards the spur gear will spin opposite of what it should. This in turn will spin the clutchbell. If you installed the FOC correctly this is probably the whirring noise you are hearing, and it is normal. After installing the FOC you should have two teflon shims left over among your other parts. One is a 8x12x0.5mm and the other is a 5x8x0.5mm. If all that you are hearing is a whirring sound and not a grinding you should be fine. You might check your spur gear mesh. If it is too tight or too loose it can be pretty noisy.