WrxSti03

I'm wondering if anyone can shed some light on a knocking sound I've suddenly got on my 03 sti.
After going for a drive earlier today as I came back to park I let the car tick over and I could hear a knocking sound coming from some where on the drive train.
I'm pretty sure it was the front diff but can't be certain, if i push the clutch in the knocking stop's but then obviously releasing the clutch it starts knocking again.

Cheers

John.