Before you change the pads in the hitch this is what to do. Firstly, you haven't stated whether you have a detachable or standard towball. If it's the detachable type all the better but if you haven't and have a fixed towball then remove the towball from the car (or if you have a spare one somewhere use that). Basically what you have to do is connect the towball to the hitch as you would normally but without being on the towing vehicle. With the towball in one hand, release the hitch lever as though you were hitching to the car then fit the towball under and up into the hitch. With the other hand bring down the hitch lever and lock on to the towball as you normally would. Take your hands away and the towball should be locked into the hitch socket as normal. Get both hands to the hanging towball then push and pull to and fro to see if you can feel any movement. If there is then you have the evidence that the pads inside the Alko hitch are worn and need replacing. If there is no movement and you can't twist or budge the damn thing then it's not the friction pads and you need to look at something else that's causing the clunk. I have an engineering background for over 30 years and this what I would do to before I spent any money as guess work could cost a tidy fortune in the end.
You're welcome, that's exactly what this forum is for and even though I have been towing for over 6 years it's amazing how much knowledge I've gained from this site. Anyway, good luck and hope you find the cure to your clunk!