Welcome to the forum, indent for a fireproof suite of armour.
If you want advice, here goes.
First off, double check the prop, you could have something nasty on it. That would account for the vibration.
Is there oil in the gearbox?
Is it totally quiet out of gear?
Is it the drive plate between the engine and the gearbox breaking up? There is no clutch there.
TMC boxes usually whine even when new but not grind.
Use a listening stick between your ear and various parts of the gearbox, engine, bell housing etc. to locate the source of the noise.
Have you recently had something hard hit the prop? Could it be bent or the shaft bent or the engine mountings torn off?
Is the engine running on all 4 cylinders?
When and how did this start? Was anything altered just before?