Is this a knock or a rattle?
When is it the loudest, when the engine is ticking over (idling) or when it is under load?
Is the noise there just when you first start the engine from cold, or is it there all the time?
Have you checked the basics like the oil level and had a look around for anything that could be making the noise -e.g. Exhaust system, Alternator (or other items) loose on their mountings?
Sorry about questions rather than answers, but if you can give us a bit more information, we stand a better chance of giving you a more accurate answer.