440 head noise. Perpetual problems. Problem solved

So my 440 has developed a head noise on the passenger side. I took off the valve cover and noting looked out of place or loose. I started it without the cover on and the noise is still there it's just louder with the cover on. The noise all but goes away when I put the car in drive or reverse. It's like a loud rattling sounds. Hard to describe. Any help would be greatly appreciated