Whirlpool AWZ3303 – vibrating/rattling noise?

Hi, for some time now the AWZ3303 has been making an awful rattling noise during it's cycle. I have it positioned on top of our washing machine in a utility cupboard and it sounds like someone blowing a monotonous tune on a didgeridoo! It works fine but I would like to eradicate the sound. Obviously, if I can order the parts needed and have them in front of me, I can fix the problem and have it back together again in one go. How can I go about identifying the faulty components and ordering them? My instincts tell me pulley wheels from what I have read, but which ones do I replace? All?
TIA
deano.