New wheel problem.

Monza Posts: 27
edited June 2011 in Workshop
Just fitted new Campag Zonda wheelset. Fitted new Centaur cassette and torqued to 50nm. Every now and then I get a mechanical crack from the rear wheel. Its a sharp crack (amplified by the ti frame) and I can feel a tiny give through the pedals. It happens in all gears and sometimes after a gear change, not always though. Sometimes after a corner, there seems to be no real pattern, apart from resuming pressure on the pedals. I've had the wheel and cassette off three times but still the problem persists. I am sure that the cassette is on right. I used copper grease on the threads prior to assembly. Could it be a faulty hub, or am I missing something? :(