The pouring of liquid into the glass can excite weak "organ pipe" vibrations of the air column above the liquid. As the column of air gets shorter, the frequency that you hear gets higher.
It has nothing to do with tapping the glass, but a similar thing can happen with the vibrations of the glass itself if you do; in that case the frequency gets lower as the glass is filled.
Original Q: If you pour liquid into a tall, narrow glass, you may hear sound with a steadily rising pitch. What is the source of the sound? And why does the pitch rise as the glass fills?
Additional question from me: What does it mean by the source of the sound? like the tapping on the glass?
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Plunger Jihads for the win!
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