It happens that during intimate relationships, the vagina releases sounds similar to body gases. But you shouldn't worry: This is something that usually happens, and it's not about the gases of the system of the third.
Indeed, it is a mass of air that enters the vagina, and then because of the pressure exerted in the vagina comes out by releasing a choked noise. Experts explain that these noises can always occur, and it is quite common, as air masses can enter the vagina at any moment.
However, there are some situations in which this is most likely to happen: during stimulation with fingers, penetration of the penis, sex toys, which push the air to enter. When you're excited, the entrance to the vagina expands, making it easier to get out of the small air masses.
Source: Cosmopolitan