Knock sound on peugeot?

Ive got a knocking sound coming from the front drivers side maybe under the bonnet or just above the tyre on my peugeot 306 which you can hear only when you drive on rough road, its fine over humps.
I think it may be the shocks, any idea on what it could be and what prices the parts are. Also if you could let me know if its a big job or not.

Thanx in advance.