Wallrus

Hi guys, just need some advice.
I have a 2005 Impreza wrx, its done 77k miles. It has recently started to make a grinding noise at the rear, like a metal on metal noise. Its not constant so I don't think its a wheel bearing, it is completely random.
Has anyone experienced this before or has any ideas what it could be.
Someone at work said diff (really hope it isn't).


Any input would be appreciated


Cheers