That weird hissing sound behind the bathroom door...

- Definitely agree, shutting off the supply and still hearing the hiss rules out a lot of plumbing issues.
- HVAC is a prime suspect - small duct leaks or even a damper not closing right can make a weird hissing, especially when the system cycles.
- Don’t forget about toilet fill valves. Sometimes they’ll hiss even if you think the supply’s off, depending on how the shutoff’s seated.
- Had a tenant once swear there was a leak, turned out to be a cracked vent boot letting wind whistle down the stack. Drove us nuts for days.
- If you can, check during different times - sometimes it’s only when the heat or AC kicks on, or when another bathroom’s in use. That can help narrow it down.

Not totally convinced it’s always HVAC or a fill valve, honestly. I’ve run into a couple spots where the hissing ended up being from a pipe rubbing against a stud - like, not a leak, just vibration when water was running somewhere else in the house. Sounded exactly like air escaping. Took me forever to figure out because it only happened when someone flushed upstairs.

Also, about those toilet shutoffs - sometimes people think they’ve closed them all the way but there’s still a trickle. But if you’re dead sure the supply’s off and you still hear it, I’d start looking at stuff like vent stacks or even wind whistling through a cracked window seal. Had one job where it was literally just the neighbor’s AC unit echoing through the wall cavity... drove me nuts.

Point is, sometimes it’s not even your plumbing or HVAC at all. Sound carries weird in these old houses.

That’s actually super reassuring to read, because I’ve been chasing a similar noise for weeks and was starting to think I was losing it. The bit about sound carrying weird in old houses really hits home - mine’s from the 50s and sometimes it feels like you can hear a pin drop three rooms away.

Point is, sometimes it’s not even your plumbing or HVAC at all. Sound carries weird in these old houses.

I kept thinking it had to be the toilet fill valve, but after swapping that out (twice), the hiss was still there. Turns out, it was just the wind sneaking through a tiny gap in the window frame. Never would’ve guessed if I hadn’t sat there during a storm and heard it get louder.

I do wonder if some of these noises are just part of living in an older place. Not everything needs fixing, but man, it’s hard not to obsess when you hear something new. Glad I’m not the only one who’s gone down the rabbit hole with this stuff.

That’s a good point about not everything needing to be fixed. I’ve definitely spent hours chasing down noises that turned out to be totally harmless - like the time I thought I had a leak, but it was just the old radiator pipes settling. Still, it’s tough to ignore new sounds, especially when you can’t pinpoint them. Have you noticed if the hissing changes with the weather or time of day? Sometimes I find certain noises only show up when the house is cooling down at night... kind of makes you wonder how much is just the building itself shifting around.

Sometimes I find certain noises only show up when the house is cooling down at night... kind of makes you wonder how much is just the building itself shifting around.

I hear you on that - old houses especially can sound like they're alive after dark. Had a job once where a customer swore there was a leak behind their bathroom wall because of a faint hiss, but it turned out to be air moving through a slightly loose vent cover. Still, I always tell folks: if the hissing gets louder, or you notice moisture or a drop in water pressure, don’t ignore it. Sometimes it’s just pipes settling, but sometimes it’s not. Better safe than sorry, right?