If the sound is rhythmic, the cause can be an unbalanced load or a foreign object in the drum. If you suspect this to be the case, stop the machine and drain the water inside. Remove foreign objects or add other items like towels to balance the load without exceeding the appliance’s capacity.
These noises can also be caused by the appliance bumping against its hoses at the back, a wall or a cabinet. Check that the hoses are securely in place and that the appliance hasn’t moved from its original position.
Lastly, check that the appliance is standing level and balanced on its feet. Try rocking your washing machine from side to side or back and forth. If it is wobbling too much, your flooring or the appliance's feet may be uneven.
Use a spirit level to check the appliance's balance. If it's off, adjust the feet by reaching under your refrigerator and finding the two front feet. Turn the feet clockwise or anti-clockwise, until they are firmly in contact with the floor.
If your appliance isn’t operating normally while making this noise, contact an authorised service agent.