First of all, you must state WHY you are changing the timing belt. If you are changing the belt due to a scheduled maintenance, then we would have to go from there, but if you are replacing a belt that has broken---well my friend------I am here to tell you that you have what is called an "Interference Engine". An Interference Engine is an engine that does not have adequate clearance between the pistons and the valves in case of a timing belt failure. Lacking this adequate clearance, in the case of a timing belt failure, the pistons "Interfere" with the valves, causing them to bend. This "Knocking" noise that you hear is more than likely the pistons banging against bent or broken valves. This can be diagnosed by doing a compression test. However, when seeking advice, next time , give the history of the vehicle prior to the parts exchange. It can help narrow your situation to the problem by 90 + %.
MR. VINCENT. ASE MASTER TECH