This is one of the symptoms homeowners are most tempted to dismiss — until it doesn't go away.
A consistent hiss, trickle, or faint rushing sound behind a wall, with no fixtures running, is typically pressurized water escaping from a small crack or joint failure in a supply line.
The sound can be faint, intermittent, or easily confused with normal house settling or plumbing 'water hammer.' The key difference is that a leak sound tends to be continuous and unrelated to any fixture use.
Our ears are decent at noticing something is off, but not at pinpointing exactly where in a wall cavity the sound is loudest. Acoustic equipment isolates the precise source, avoiding unnecessary drywall removal.
Normal pipes can tick or pop briefly with temperature changes, but a sustained hiss or trickle with nothing running is a different pattern worth investigating.
Reasonably quickly — a wall leak left running can lead to structural and mold issues well before it becomes visible.
Let's get you a real answer, not more guessing.