Nasty rattling/knocking noise, 98 LSS

My supercharged LSS is making a scary knocking noise at and below 1100 rpms, it gets worse when i put it in gear and just sit there with the brake on. I've removed both belts and tested all the pulleys, they're all solid. But when we try to turn the motor a bit manually by spinning the either belt while attached, the main crank pulley only goes so far in either direction, clockwise or counter, and clanks to a stop, making a similar noise to the one it makes while running. (We took the main belt off and turned the motor a bit with the other attached, and also with did the same removing the supercharger belt.) Note that the noise fades out quick as the rpms raise in gear or not. Could this be my harmonic balancer? And if so, what does that entail to fix it/replace it?

So you cannot get one revolution manualy on the engine?, unusual. And yes the balancer could cause this, try and move the outer part of the pulley or look closely while running you should see some indication of looseness on the outer diameter. Can you turn the engine freely with no belts?