Hi, i've just got a Bauhaus walstein alto sax that is a few years old. The lower stack keys [F<E<D} are somewhat noisy compared to a new alto that I have to check against. I find that if i put a folded tissue between the felts and the body then that quietens it considerably. Do these tiny felts [ they are on the ends of adjusting screws ] tend to go hard? What s the best fix please?