Beko DRCS 68 W - rattling noise when drum rotates?

My tumble dryer has started making a rattling noise when the drum is rotating. First of all I thought it was a coin going round inside the drum as that is what it sounds like but I took all the clothes out and checked inside the drum but found nothing. I switched the machine on whilst empty and it still makes the noise. The machine heats up and drys ok. It sounds like something has stuck behind the drum.
Is it possible to get into the insides of these machines?