[Fast Corners]

Pictures would be helpful but it sounds like something is funky with your valve spring assembly. As said above the springs are pretty stout but if one of the two or both are broken or are the incorrect part then that may be where the problem lies. There are other pieces in the system that may warrent further inspection as well. There are "rings" and "washers" (read - spacers) that are used to set the valve spring height. If the springs are ok it is possible that these may be missing or assembled incorrectly. In effect there would be very little to no preload on the spring which may explain why you can depress the spring with hand pressure.

From the sounds of it this would explain your mysterious knock very well. With little or insufficient spring pressure your engine was probably experiencing valve float. At low rpms the spring pressure was sufficient to keep the valve "on the cam". At higher rpms the valve may have "floated" in which you may just have heard the slapping of the valve. In a worst case scenario the valve may have contacted the piston dome. Stock SC cams are pretty mild so hopefully there was no piston/valve impacts.

Depending on what you find out with regards to what was wrong with #5, I would recommend checking the other valves/springs pretty closely before butting everything back up.