I have just replaced the pinion drive gear. It looked like it had demolished itself into the ring gear under heavy brakeing. I did wonder about that noise at the time.
The car was on a chassis dyno the other day, and at high revs under load, the new drive gear just vibrates out and into the ring gear and makes a lot of sparks.
I had strenghted up the return spring before hand, and polished up the bendix (helix) to make all slide better, but still have the problem.
Any suggestions other than giving it the float test?