I own a 1996 Ford thunderbird 4.6L v8 (192,034 miles) with over head cams (one over each head) I recently had a problem with lack of power/miss fire under load, this problem is fixed, I replaced the plugs and the plug wires. While I was trying to diagnose the problem above I noticed that the engine had an intermiten knock coming from under the passenger side valve cover, I removed the valve cover and could not find anything that was loose or that looked broken, when I had the valve cover off and started the engine, I could no longer hear the noise. When i replaced the valve cover the noise returned. The noise seems to stop when I give it gas, I can only hear it at idle (or its at least louder at idle). What could be the problem?