My Bosch WAA28165GB is sometimes really noisey on spin?

Sometimes my bosch washing machines is really really noisey on spin and sometimes its not. It sounds like there is something in the drum, but how would i get inside to find out. I have empties the drain plug/filter and there was nothing in it, it sounds like a coin or something rattling round. Put it on a wash with no washing in and it didnt do it.
Please help, its frightening my dog and hurting my ears!
Gill