abnoxious knocking noise

i recently installed a completely rebuilt small block 350 into my monte carlo ss. This motor sat for a while before i was able to install it. It is a 4 bolt main, i got the crank cut, block boiled out, i put a mile street cam in (blue racer), got the block bored .30 over, got the springs to match the cam, had a edelbrock performer intake manifold i used, headers, 2 and a half inch exhaust. It fired up, and seemed to run well at first. It developed a slight 'tapping' noise that we first thought was a valve. We adjusted the valves 5 times, and the noise still remained. I then grounded out each cylinder, the noise still remained. So my thoughts were that it was upper end. I then began to take apart the heads, staarting with the rocker arms, then push rods. I inspected them, they all seemed fine. I then checked out the lifters, they were all fine. I am going nuts with this noise.. Any suggestions would be greatly appreciated. Oil pressure at initial startup is about 60, then at running temp is about 15-25. Idles good, no misses... any one any suggestions???

Check the Flywheel for hair line cracks.