Can you reproduce the noise when the car is stationary, or only when it's moving?
If it happens all the time, even when the car is standing still;
Have a look under the cam-belt cover and make sure that your cam-belt tensioner is still holding the belt tight and that the belt is central on the pulleys (not running off the pulleys). Grab hold of the water pump pulley and give it a good pull up and down to see if there is any play in the pump bearings.
While you're there, have a look at the flywheel pulley to see what condition the rubber insert is in. The pulley has a rubber ring bonding the pulley part to the central/crankshaft part (like a sandwich) and the rubber can deteriorate.
Check the exhaust heat shield to make sure it's not that, banging on something.
If the knocking isn't a metal-to-metal knocking... just more like a fast tapping sound, then check your spark-plugs are not loose and that the coils are in good condition. These would cause a misfire or hesitation and you don't mention that symptom.. but it's worth mentioning.
Do you have any octane booster? It could be your petrol is just rubbish/full of water and so the car is pre-detonating. Get some more octane in the tank... fill the tank with some new petrol from a different garage or stick a whole bottle of octane booster in there. Your garage might have sold you parafin or something.
If it only happens when the car is moving:
Does the knocking sound like something going round and round (knocking gets faster when the car is travelling more quickly) or does it knock at the same frequency?
Check the condition of the driveshafts. Jack the whole front end up (put each side on stands) and pull the driveshafts in and out of the cups. If there is more than a few millimeters of movement, then it may be wear in the inner CVs. Does the car "rumble" when you go up hills or when the road dips (so the suspension is compressed)?
It could also be the clutch. How many miles has the clutch done? Does the clutch judder or can you feel the knocking as a vibration under the clutch pedal, or is it just a noise?
Ralf S.