Well this does sound familiar. Very probably a broken weight (lumps of concrete bolted to the top and sides and possibly the bottom of the drum)
Slide off the top of the washer, two screws should be all there are to take out. (unplug it first!) and take a good-long look. If it is the concrete, take out the loose bits and if necessary, unbolt the blocks. (Socket set is usually needed here) You should put another weight back, but if you load carefully and take it gently, it will be fine.
James, May 2011
