Vibrating sensation whilst Mediating.

[Unbound]

You are experiencing the organization of the energy of your chakras by higher vibrations, the activity felt is the interaction between the different vibratory levels as they are "evening out" or equalizing. Continue to meditate in this way and likely the sensations will change as your system is balanced and organized. In some cases it may indicate blockages being worked on and opened up by incoming energy fluxes.

Also, to your comment GW, I would say that while our actual yellow-ray bodies may not directly detect 4D or other density energies we do pick those energies up with our astral/emotional body and further with our mental and causal bodies.