I just scrolled straight to the bottom to answer this. Sorry if it has been answered already.
It's the differential. It's normal.
The mounts and/or bushings are probably not the best. Personally I like the sound..... Because Racecar
I have several friends with 240's and other cars similar to this that all make the diff clunk noise.
Edit: I watched the video: obviously not the diff lol. But our car does still make that noise in the diff, for manuals.