Knocking is usually caused by slow ignition or because ignitions is too soon or too late. One of the easiest things to check is to make sure the gap on your spark plugs is at manufacture's specs. Depending on the mileage, another issue you can have is your camshaft and crankshaft are out of time. This happens when the timing belt (which is made of rubber) has more slack than the tentioner can take up. One way to check if its a head gasket, look for oil in your coolant or water in your oil (wierd spots in coolant or a white-ish looking sluge or goo in oil).