Why does dryer rattle on clockwise rotation only?

I have a Creda Simplicity tumble dryer and it makes a clicking/rattle noise whe rotating clockwise but OK when going anti-clockwise,I have removed all coins etc from filter and outlet pipe as well as loads of built-up fluff internally still have the rattling but only when drum turns clockwise,before cleaning made noise both ways,anyone shed some light on my problem?