Sounds just like something I just went through. Turned out to be a rod bearing. It knocked on start up but not when it was just sitting there idleing. Then it got worse when it got warm. Then when I drove it got louder. Hopefully this isn't the case with yours. But fortunatly I was able.to find a good deal on a block from the boards! Another thing I remember was good oil pressure at start up but it dropped lower as the engine warmed up.
