318 Poly Vibration

I used the torque converter on the trans that was in the car. The trans works extremely well, the kickdown o-ring was hard and leaked every so often so I replaced. It's just a rubber ring. Changed fluid, no other leaks.

What are you thinking?

Engine mounts are new, have checked the mounts, bolts, made sure they are tight. They give a very small amount if you rev it, seems normal.
Trans mount cannot get, everyone is out. Not the same as later mounts. One guy makes them, but many have issues with paying and never receiving.
Laid under car, trans was put in and out of reverse, no movement, nothing obvious, it's doesn't look dried out. Tried to get one again today, no luck.
Any other way to check it?

Recorded with phone, which fell over at end.