This is an excellent article explaining the critical period for turning eggs:
https://thepoultrysite.com/articles/the-biology-behind-egg-turning
I'm looking forward to hearing what hatches from your mystery egg.
The duckling isn't actually tapping - the sound you hear is it transitioning from relying on the vascular system for oxygen to breathing for itself. It could be internally pipped, or close to it. Can you candle it to see how it looks? If it has internally pipped you'll see its bill distinctly...