Well, I think it's another indication that the whole thing can never end in a way that will satisfy the Master - whatever is "wrong" with the Master, whatever his "itch" is, it can never be "scratched", so he'll just go on and on forever, getting ever more desperate and more mad...

I maybe don't really wholeheartedly accept RTD's idea of the drums in the Master's head, but it's certainly a good metaphor for the above - the never-ending quest for the impossible that he follows.