Glad your problem was easily identified. Now it's time for a little research.
QUESTION: why would you expect a marble slab to isolate a TT (or any object) from vibration? Would a car manufacturer install marble blocks as shock absorbers? How would the car react when driving over bumps? Small ones would rattle your teeth loose, big ones would launch the car airborne.
The only way to ISOLATE an object from vibrational energies is to eliminate physical connections between the vibration source and the object. Placing your TT in a vacuum chamber that's not in contact with any vibration source would work, but that's impractical without ready access to a space station. (Of course a marble slab does the just opposite: it CONNECTS the object to the vibration source.)
Practically speaking, the best we can do is ATTENUATE vibrations before they reach the TT. Vibrational energies are attenuated when we allow them to generate movement:
- OF the structure(s) between the vibration source and the object, like the springs on a car (but a marble slab is too heavy to move); and/or
- WITHIN the structure(s) between the vibration source and the object, like Dr. Scholl's gel insoles (but a marble slab is too dense and brittle to flex).
The material properties of marble (dense, very brittle) are the opposite of what's required for the attenuation of vibration. That slab is actually making your problem worse. Before you buy anything, ditch it.
