[dnc]

I can experience something similar with the ventilation system in my apartment. Usually, the sound of it is immersed in the background noise together with other sounds coming from the outside, so most of the time I'm not aware of it at all. However, sometimes when I meditate I hear it and if I focus my attention on it, after a while it becomes clear and loud, rich and versatile, not unlike a music played on a wind instrument.