ken, a bight is a sagging section of a loose rope or belt - hence the "Great Australian Bight" on the south coast is the shape of an upside-down sagging rope. Slack belts flop about and fall off their pulleys.
When your pulley moves up and down, is it moving axially, or is it rocking? Also, if it is rocking, is it because the pulley shaft is loose in its bearings, or because the whole housing is loose on its mountings? The causes of the these three conditions would be different.
