Please see the video link.

I've poured hot chocolate milk into the glass. After stirring I start hitting the bottom of the glass near its wall at the same point. The frequency of the sound is going up.

I'm not sure about the explanation, what could it be? I've observed it multiple times.

http://www.borgulya.hu/~gabor/20140924160920BST%20Glass%20with%20hot%20drink,%20hitting%20bottom%20%28c%29%20Dr%20Gabor%20Borgulya.mp4