Turning off the main and draining the lines is definitely the safest way to start narrowing it down—props for thinking of that first. I’ve seen a lot of folks jump straight to cutting into walls or floors, which can get messy real quick if you’re not sure what you’re looking for. Sometimes those hissing sounds are just air trapped in the system, but other times it’s a slow leak that hasn’t shown up as water damage yet. Did you notice any changes in water pressure or see any moisture around the baseboards or fixtures? Sometimes even a tiny drip behind a wall will make that noise, especially at night when everything else is quiet.
One thing I’d add—if you’re in an older house, sometimes the pressure reducing valve (PRV) starts to fail and you’ll get a constant hiss even if there’s no obvious leak. Ever had any plumbing work done recently or noticed the sound gets louder after someone uses the shower? Just curious, since those little details can point you in the right direction...
I get what you’re saying about the PRV, but in my experience, those usually make more of a whistling or vibrating noise than a straight-up hiss. Had a similar issue last year—turned out to be a tiny pinhole leak in a copper pipe behind the drywall. No water stains, just that weird sound at night. Have you tried putting your ear right up to the wall to see if you can pinpoint the spot? Sometimes it’s way louder in one area than another, which helped me narrow it down before I started tearing things apart.
That’s a solid approach—using your ear to track down the noise is underrated. I’ve seen pinhole leaks do exactly what you’re describing, especially with older copper. A couple things to check:
- If you can, shut off water to just that section and see if the sound stops. That’ll help confirm it’s not something like a vent or HVAC.
- Sometimes, even a super tiny leak won’t show stains for ages, especially if it’s evaporating behind the wall.
- Thermal imaging cameras (even the cheap phone ones) can sometimes spot a cold patch where water’s escaping.
Don’t feel bad if it takes a while to find. These things can be sneaky... I once spent two days chasing a hiss that turned out to be a cracked solder joint behind a vanity. You’re on the right track.
That’s actually super helpful, especially the bit about thermal cameras—I always thought those were just for pros, but maybe I’ll try one of those cheap phone add-ons. That said, I’m still not 100% convinced this is a leak. Here’s where I’m at:
- The hissing is there whether the shower’s been used or not. Doesn’t seem to change if I run water elsewhere in the house.
- No visible stains or dampness, and trust me, I’ve poked around every inch of drywall I can reach. Even pulled up a floor tile just to check underneath (which, by the way, was a pain).
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Tried this—turned off the bathroom supply line and...hiss kept going. So now I’m wondering if it’s something else entirely. Maybe HVAC? Or some weird vent thing?“If you can, shut off water to just that section and see if the sound stops.”
I get that leaks can be sneaky and all, but part of me feels like this might be one of those “new homeowner paranoia” things where you start hearing every little noise because you’re on high alert. Or maybe my pipes are just haunted—wouldn’t be surprised at this point.
Still gonna keep an eye (and ear) on it. If it gets worse or I spot any water damage, then I’ll call in a pro. For now, just adding “mysterious hissing” to my ever-growing list of stuff to worry about...right up there with “why does the fridge make that clunking noise at 3am?”
Honestly, you’re not wrong to be a little suspicious of every noise when you move in—happens to the best of us. I’ve seen people chase “phantom leaks” for weeks before realizing it was something totally unrelated. The fact that you shut off the bathroom supply and the hissing kept going is actually a pretty solid clue.
“So now I’m wondering if it’s something else entirely. Maybe HVAC? Or some weird vent thing?”
That’s exactly where my mind goes too. Any chance there’s a vent stack or an air admittance valve tucked behind that wall? Sometimes those can hiss if there’s negative pressure or a faulty seal. Or, if your HVAC runs through that area, maybe a small duct leak is making the noise—those can sound surprisingly like water under the right conditions.
You’ve already done more detective work than most folks would (pulling up tile? Respect). Keep trusting your gut, but don’t beat yourself up if it turns out to be nothing serious. New houses have a way of making every creak and hiss feel like an emergency... until they don’t.