Knocking noise coming from driverside valves

I have a 1992 Chevy Suburban 2wd 5.7 with 206,000 miles on it. The truck has been sitting for a year and I just started driving it again. The engine sounded fine. Then 2 months later on a short trip I heard a low tap coming from the engine. I cut the car off and cut it back on and the tap became louder and when I put it in drive and proceeded to go it got louder and started to sound like a metal ball rolling around on acceleration. When it is idling it just knocks. I changed my oil and put an engine treatment in but it still sounds the same. When I put it in drive the knock goes away but as soon as I drive it does what I said up above. In neutral it sounds just as bad as it does in Park. I had no warning of this problem. What could it be? Help!