It's because of sound speed in a liquid with dissolved air bubbles. The sound speed in bubbly water can be much lower than either the water or the air alone. In acoustics this actually has the name "hot chocolate effect" from the exact situation you're observing.The pitch that you hear is caused (at least in part) by the fundamental frequency of the water column in the gl. This frequency depends on both the depth of the water and the sound speed in the water itself.Sound speed depends on two factors, density and compressibility. Greater density or compressibility both cause sound speed to decrease. Air alone, the density is low, but the compressibility is high, leading to a sound speed of around 343 m/s. For water, the density is high, but the compressibility is so low as to be almost non-existent, leading to a sound speed of around 1450 m/s.The combination of water with air bubbles is weird though. The water contributes a lot to the density, but the air bubbles contribute a lot of compressibility. As a result, you end up with a mixture that is both very dense and highly compressible. This results in a very low sound speed, as low as 10 m/s. Much much slower than either water or air alone.