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.