take off ALL the belts... remove all tension from the pulley/ballancer, and see if its mobile...
the balancer is made in two sections... the centre section which attaches to the crank, a rubber damper middle part, and the outer ring which includes the drive channels.
it would be VERY hard for the inner part to be loose and not have the outer part loose also... ie - the whole pulley is loose - this is a major problem if youve been driving for a while as it will destroy the engaging section of the crank, and the pulley will never retighten.
so yeah, de-tension the pulley, see if its mobile, get a socket onto the pulley nut, see if its loose.