A possible source of the noise is a broken crankshaft between the rearmost pistons and the rear main seal. When a crankshaft breaks, it breaks along a 45 degree angle across the shaft. This allows the motor to spin independenly of the output shaft creating the knocking sound. It explains why the knocking gets worse as loads increase going uphill. It also explains why the noise goes away when the clutch is pushed in. It also would allow the vehicle to coast backwards when in gear while making a clunking noise. I have seen this happen on a Chevy pickup once before and it showed the same issues.
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