Haha, sorry again. I figured when you said you'd heard it, you meant you'd heard it cheeping -- which is also known as peeping. But yes, tapping is another sign of internal pipping. What kinds of duck eggs are these? I know with calls, sometimes they will internal pip and then fail to external pip because of their beaks. I'd advise you to make a larger hole and check out what's going on in there if there is no external pip soon.