The sound from a noisy sheath?

i actually just asked a question like this about a week ago. ive been told its just air going up into the sheath. since both geldings and stallions have a sheath, yes it can happen to both. it is just a natural noise. some answered my question saying it needed to be cleaned, but that's not true. the noise still happened after my gelding got his cleaned, so its definitely just air not cleanliness.