Gears & Axles - Getting a Slight Clack noise from the Rear.

Ive had this problem of the rear making a funny clack noise when driving normal.Every other time I get this clack or clunk rear end noise.My car is somewhat stock.No gears,no stall etc.I could understand the circumstances of dumping the clutch way up top and letting go,Or Stalling with a big converter and letting go,but my car is stock.Im no rear end expert but seems this rear end of my has slack.Why? :confused:

maybe a pinion bearing thats bitten the dust and causing excessive backlash