There are three phenomena that occur before vigorous boiling of water that produce sound.
1) Air dissolved in water on heating forms small air bubbles at the bottom of the container. These air bubbles get released from the bottom of the container on reaching a sufficient size. The process of release produces a sound of frequency ~ 100Hz.
2) On boiling, small vapor bubbles get produced at the bottom of the container and also produce sound of ~ 100 Hz on release. However, they cool down before they reach the surface of water and collapse. This collapsing produces a sound of frequency ~ 1 KHz.
3) Collapsing vapor bubbles agitate the water to release small micro air bubbles from water and also from the air trapped in the vapor bubble. This production of micro air bubbles produces a sound of ~ 35 - 60 kHz.
I guess you were talking about either the first or the second case.. Both of them occur before you observe vigorous boiling of water and you can hear them.
There was an interesting problem posed at APHO 2008 which is the same as your question which estimates the frequencies I just quoted. It also contains references to the experimental measurement of these frequencies. I hope you will find it interesting to solve it than me answering your question. http://apho.phy.ntnu.edu.tw/download/past_APhO_roblems/2008%20APhO/APhO2008_theory_prob1.pdf
You can also find the solutions here in case you need help on this