After an engine swap, we were moving between gears to get the fluid through the whole transmission and in reverse it makes this horrible knocking noise. I have attached a video of the sound to this thread so you can hear exactly what's happening. This is only in reverse, not drive, neutral, or park, and it seems to be intermittent. flexplate doesn't appear cracked and the torque converter is up against it with all three bolts in. The mounts look good and the bellhousing is flush with the engine block. Any ideas what this sound could be? I've never heard anything like it.

Edit: It does go into both forward and reverse gears with no slip. Everything is engaging and moving the car as it should apart from the noise.