Well if we're talking about the piston rod. you may be able to pull the oil pan then have access to the rod bearings. If the bearings are shot that maybe your problem. I must warn you that knocking can come for alot of things in a over head cam engine. My first car had a 2.3L DOHC Quad4 FWD engine. It developed a mistery knock as well. I took the car to a mechanic and he told me to check a few things. Crank walk, rod knocking, cam shaft walk, bad lifters or a cracked flex plate(it was an automatic). Me and my dad replaced or checked all of these things and never fixed the problem. I wasted a lot of time and money on a wild goose chase. If the engine has a lot of miles on it just swap the motor. Its a lot easier and may be cheaper in the long run then pulling apart an engine in the car and never fixing the problem.
I hope what I've told you well save you from the headaches and stress of possible wasting your time. When I was done I was ready to bash the designers of that engines heads in.:bash
Good luck with whatever you decide to do. I know what its like.