Re: hotpoint aquarius 1200 wm 64 rattling on spin
More than likely it is a coin or something under the inner drum. Use a torch and look through the holes in the drum (go on stick your head in the door like I do!) and try and locate it. You may be able to coax it out through the gap between the door seal and the drum. Or push it into the filter trap in the sump hose and out of harms way.
Worse case would be the heater come loose. Either way may need a drum strip down to remove or investigate. Not really a diy job.