I am fairly sure it is not the bevel box banging (I have the flexy mounted one on the later chassis), as I am familiar with that noise when I creep over the very uneven and rocky surface of my car parking area. It is a "twang" sound combined with a lurch, which I understood was the belt skipping a tooth. It only happens at max torque in second gear. In sympathy to the drive train, I never get near to full throttle/max torque in first. I will check the belt tightness tomorrow, where I believe, I should just be able to twist it through 90º at mid point.
I know Vee belts stretch when they get hot, so I would expect toothed belts to behave in similar fashion. When I used to work as the engineer at the family textile mill, where there were hundreds of vee belt drives of anything up to 10 belt wide drives for the larger machines. We had to use a Venner supplied table to get the correct drive specs and belts, dependant on transmitted power, distance between centres and ambient temperature. Some the drives were inside steam drying machines where the temperature from the 200 psi high pressure steam pipes would be 195ºC. We had to use special material belts there.
Wilson
