What causes disturbances in our meditation once we have established serenity and are not throwing any more pebbles into the pond of our mind?  It’s our old stock of unwholesome mental states which shake our heart base. This happens because our field of perception opens and negative mental states reveal themselves which are below the threshold of our normal awareness. We know these mental states are just below the surface but usually distract ourselves from feeling them - maintaining a more limited level of consciousness.