Lot's to think about. Here is one more...
I have the same boat, same problem. Turns out the plate damper, which is essentially a shock absorbing plate between engine and transmission was worn. Lot's of info on the Catalina owners site including pic's on how to change it( relatively simple).
When I replaced mine, I noticed that the spline gearing on the damper was worn, which would result in the damper plate vibrating when the engine wasn't under load, (low rpm...). This would occur because the spline was no longer snug and the wear allowed for some slack between the teeth. Thus all fine when under load backward or forward keeping teeth firmly together, but when not under load, like at low rpm, the teeth don't mesh tightly, and thus you get "rattlely vibrations"