[Boog]

It is possible a small bolt, washer or nut fell down into one of the intake runners of one of the heads and is sitting on top of the piston. If not very big it can get pounded into the piston top without bending a rod or cracking a cylinder. I have found a few over the years. Also a loose torque converter bolt can cause a hard knocking sound.
You could use a mechanics stethescope or a long piece of hose to helpl narrow the search for the source of the sound. Good luck with it.