I have a modified 98 lude that is knocking. It starts at about 2500 rpms and as the rpms rise, the knocking gets faster and louder. The sound is coming from the top and front of the engine. I inspected the cam lobes and readjusted the valves. Then did a leak down test. All seemed well. I cranked it with the valve cover off and the knocking was gone. put the engine back together, and it is back. I don't know where to go from here. Any idea's?

If you aren't dead set on it being from the cylinder head then I suggest it's a rod knock/bearing failure. Sometimes it sounds like its from above and isn't. Get a long screwdriver and put it to your ear and touch it on different places to pin point the noise there is also stethoscopes for auto repair which will make you not look so weird when you do this.