dryer making a knocking noise ?

The drum insde that rotates, is usually held in place by three or four big springs or rubber shock absorbers so that it can move a little but not too much, and if they have come loose or got old, the drum will have more freedom to move about, especially if all the clothes are at one point in the drum. Sometimes there is an adjuster device which consists of two nuts on a threaded bar that can be unlocked from each other, move along a bit then tightened together again. other types of adjuster exist but are usually more difficult to adjust, and some don't have and adjustment. If the maximum adjustment has been reached or there is no adjustment, you will probably need to change the springs/shock absorbers. please do not proceed to open the unit unless it is outside its guarantee period as doing so whilst still under guarantee may void that guarantee. If it is still under guarantee, phone the seller and describe the situation to them, they have almost certainly had this situation before and should repair or replace it.