Help! Small block 360 has a knocking sound

Cudafever, could still be that the Flex Plate bolts are tight but a tad too long bottomed out in their threaded holes in the converter.

This senario the flex plate could still walk on the bolts at the converter producing the knock.

Next test, put some flat washers on the 4 flex plate bolts and see if that tightens the flex plate to the converter.

Now start it up and test again and see if the noise/knock goes away. If it goes away, tells you wrong length bolts mounting the flex plate to the torque converter.