This is slightly related to Bri’s question. When I’m sitting quietly on the can
in my upstairs bathroom I can clearly hear the sounds from outside, almost amplified. Particularly loud is the engine noise from an Avanti that passes over at a fair height once a week. The Army choppers (Sioux) pass regularly over at around a thousand feet, filling the room with sound. Also coyotes yapping in the fields across the way, yipping pups especially are amplified. Canada Geese honking on the river about quarter of a mile away, come in strength five.
Could it be the vent pipe as in Bri’s case except no mushroom, which of course is open to atmosphere (apart from the extractor fan) I hardly have a stealth bathroom. it’s actually more like an echo chamber. I’ve often wondered if sounds from my bathroom do not flow in the other direction also, and are broadcast throughout the neighbourhood. Some of my neighbours are not terribly friendly. I always thought it was my accent….