Hey,
Thats really weird, but I believe you. When I was younger my mother and I moved into this new house, and sometimes when I would walk down the hallway I would hear someone strumming on a guitar softly. Like when you pluck on an electric guitar without an amp. It would continue for a few seconds after I entered my bedroom, then stop. I lived in that house for over six months and it happened almost every day.
Maybe the ventilator creates a kinnda echo chamber that projects frequencys that you would not be able to normaly hear.