I'm sorry I really have no idea, but are you sure its comming from the wheel well? Have you checked the other things in that area. Within the well, how's the wheel? Is it worn? Is the rim bent? Is the paneling inside the well loose? What does the knocking sound sound like exactly? Like metal on metal? Or is it more of a thump thump sound? How are your brakes? Rotors... pads... caliper...? do you have sport suspension? After market?
I'm no mechanic, I'm just trying to open this up incase anyone else knows of your situation.
Just incase you have something similar to what I've experienced... I have Dinan's Stage 1 suspension kit installed. When I'm going slow, making a turn, I hear a sound from my suspension that sounds like something is rubbing, though it's not a knocking sound.... Dealer told me that it's natural from the Dinan suspension components.