First make sure machine is level, on firm ground and no knocking against anything. If not this -- Don't know what make you have but this could be caused by broken or loose weight used to hold drum and stop it moving around on spin. If weight is made of concrete it may have cracked. Remove to of machine to see if top weight cracked/loose. Move machine out to se if concrete dust underneath for bottom one. If iron weight - may have come loose. If the cause is the weight - STOP using and repair (£50 ish for new iron weight) as drum moving will damage seals and bearings - more expense and leaks.
Alfie Webb
January 2005