1999 Chevy Malibu Engine Noises

Engine problem
1999 Chevy Malibu V6 Front Wheel Drive Automatic 151877 miles

The 99' Malibu has had an engine leak for at least 4 months that I know of. It started to make knocking noises awhile ago, not to long ago. One night I was going up a hill and it started to knock really loud. I only had like 7 blocks to go, so I continued and then I heard a noise as if something fell out the bottom of the car (thump). When that happened, the knocking noise immediately lowered (not all the way). I am on a really short budget, so automatically I have to it myself. Some people have told me that they think it's y cylinders and some have said that it may be my rocker arms or spark plugs. I know I can fix it (moderate experience), if only I could have some confident direction of where to look or what to fix. PLEASE HELP if possible.