Its the lifters one of them are collasped its an easy fix if you know engines otherwise it will cost about 400 dollars from a mechanic A lifter makes a tick. If it is knocking loudly, then my money is on a spun rod bearing, also known as rod knock. I would also be willing to bet your oil is way too low, since oil starvation is the quickest way to ruin your engine bearings. The motor will probably be quicker and cheaper to replace with a good used one than to rebuild. If you keep driving the car, it will "sling the rod" through the aluminum engine block, sieze up, and leave you stranded somewhere.