There's supposed to be a rubber stop which prevents what you're hearing. It fits into a hole in the metal piece the clutch arm hits when fully pressed (get down there with a flashlight and watch as you press the clutch in with your hand). If you don't see one there anymore (and rather you see a hole), yours probably broke in half and fell on the floor never to be seen again. Mine broke in half too and I noticed it when I was down there inspecting the clutch spring.
You can replace it with just about anything rubber or felt that you can find laying around with an adhesive back. As mentioned above, @
Ultramaroon found chair leg pads to be a great replacement. Since I didn't have any and cbf to go get some I used a cut piece of rubber drawer liner and 3M tape. Let me know if you want a picture.