It sounds as though something flimsy (plastic ?) is trapped between the drum and the outer wall of the White Knight Servodry machine. How do I access this ?
Open the door and look at the back face of the drum wher the screws or rivets are. Rotate the drum using the paddles and watch the area around the centre. You may see a crack appear as the metal flexes and bangs as it falls back into place as the tension is released.
My high speed gas one does just this and is only 21 months old!
Jeremy Downing
May 2007
It sounds like the outer shell of your drum is cracked
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Try fixfault.com to fix tumble dryers.