Sounds like you have a rod knocking this an expensive fix but if you don't fix it you can throw the rod which will permanently damage the engine beyond repair. You can get additives to silence the knock who knows how long it will last. The rod is a long piece of metal that attaches the piston to the camshaft. The spark plugs fire push the piston down on the rod wich rotates the camshaft or turns the engine which in turn sends power to the transmission wich sends that power to your wheels. If it's more of a clacking sound like several people drumming with pens on a engine block then it could be your oil if your lucky. If it's the rod you might star looking for a new car, it aint gonna bs cheap.