Get a noise locator stheoscope and listen to the top of each valve cover approx above each spark plug. If it's audible while you are standing in front of the car, you should be able to pinpoint the source, or at least get it narrowed down a bit. I hope it's not a connecting rod or a piston slapping or a bad piston pin. You didn't mention much about the quality of the "knock", ie, is it high pitched or low, does it vary with rpm or load, can you hear it inside the car while driving??? How many miles on the car, how is the oil pressure? Lots of possibilities here..
[This message has been edited by jeffsr (edited 01-28-2000).]