Noise from the rear of my car
 

A while back i noticed a whining sound from the rear of the car 93wrx. Sounds like a dodgy wheel bearing sound.
It happens when in gear and lifting off the throttle (engine breaking).
I have noticed it in 3rd and 4th, in 4th gear it is when the speed is 35mph - 30mph whilst slowing down. In 3rd its a bit slower but not too sure.

I thought it may be the rear diff so i changed the oil to castrol syntrax, the old oil was pretty thick and black but there were no metal bits in it or on the magnet on the drain plug.
This hasn't made the sound go away.
Is there a bearing that it could be? if so could i regrease it or do something?
Should i be worried??

Many thanks
Matt