Triumph Repair: 1976 Spitfire differential noise, rear drive shaft, wheel hubs

Question
I have a rear end knocking noise with frequency proportional to wheel speed.  It only occurs when power to the wheels is off.  With power on (acceleration) it is quiet.  Hard braking also makes it quiet.  By simply applying slight gas, the noise stops.  I have just replaced both rear drive shaft u-joints with no improvement.  Is it the differential?  Propeller shaft?
Thanks

Answer
Hi Jeff,
If the noise is with wheel speed it can not be prop shaft as it spins about four times as fast as wheel speed. Since you already put the axle "U" joints in, then that only leaves the outer axle wheel bearing outer race problem (roller bearings out at the wheel hubs run directly on the axle)
If it were a pit in the axle it would do it under load or on decel as would a broken roller so the outer race is the only suspect out there. However, a chipped tooth on the ring gear in the diff is also suspect. A chipped tooth on the pinion would be at prop shaft speed and the spider gears are not rotating when going straight so they are not suspect.
You may try to listen to the outer axle bearing housings with a stethoscope while on a lift and listen to the diff housing too with the car in high gear and maybe even a little load with the brakes to see if it can be more closely IDed. If you have any wheel bearing play it can even be a brake shoe contacting the brake drum under load.
Either the diff or the outer wheel bearings are a lot of work each. It would pay you to try to better pin point which it is before taking it apart. Just removing the wheel hub from the axle is sometimes difficult to do without warping the hub.
Howard