It has been known for some years that the Knudsen region of the ambient noise spectrum is associated with breaking waves. Recent work has shown that this is the case even at low sea states, when there is little or no observable whitecapping. It has also been shown that the main mechanism which produces the noise is free oscillations of bubbles. In this paper, we describe some experimental results which seem to confirm these facts and to refute various alternative theories involving spray impacts and turbulent forcing of bubble oscillations. The results also show that the mechanism which excites the bubbles is their formation at the surface; once a bubble has been formed and has radiated the excess of energy resulting from its formation, it is more or less silent. It is possible for extremely violent conditions to re-excite bubbles by breaking them up into smaller fractions, but it is not clear how important this process would be in the ocean. Having shown that bubble formation is probably the predominant source of the Knudsen spectrum, we then discuss how the entrainment process imparts energy to the bubble; the conclusion being that the energy comes from radial flow around the bubble at the time of formation and that surface tension effects are probably not important.