Hypothesis 1 : The sound is coming from the neighbours either upstairs or downstairs as the first instance of the sound heard by me is from the outside of the windows. What the maid heard was also sound coming from outside and she thought that the sound is coming from within the room itself due to rebounding of the sound waves. My sis didn't hear the sound because she has blocked ears
Hypothesis 2 : The sound is coming from an external source other than neighbours but has a sound which is similar to a person exhaling. My sis didn't hear the sound because she has blocked ears
Hypothesis 3 : The sound is emitted from a ghost who only allows me and my maid to hear it, but somehow did not allow my sis to hear it