You can hear it, but you can't find it. A hum in the wall. A buzz that comes and goes. A low drone you notice at 2 a.m. and nowhere else. The problem with a mystery noise is that your ears are great at hearing it and terrible at telling you where it is or what it is.
The fix is to stop treating it as "a noise" and start treating it as a frequency. Nearly every steady sound sits at a specific pitch, and that pitch is the biggest clue you have to its cause. Here's how to read it — and the one step that actually pins down the source.
Step 1: Is it low, or is it high?
Before any tools, just characterize it by ear. Low sounds and high sounds come from completely different worlds:
- A low rumble or drone (you feel it as much as hear it) is almost always mechanical — something is spinning, pumping, or vibrating: a furnace blower, a fan, a pump, a compressor, a motor.
- A steady mid-pitch hum — the classic "electrical hum" — is usually electrical: it's tied to the power line.
- A high whine or squeal points to something electronic or fast-spinning: a switching power supply, an LED driver, a failing bearing, coil whine in a device.
Step 2: What the frequency tells you
Once you can put a number on the pitch, the diagnosis narrows fast. This is the rough map:
| Frequency | Usual cause |
|---|---|
| Under ~150 Hz (low rumble) | Mechanical — HVAC blower, fan, pump, motor, compressor |
| Exactly 50 or 60 Hz (steady hum) | Electrical — the power-line frequency itself (60 Hz in North America, 50 Hz in most of the world) |
| 100 / 120 Hz, 180 Hz… | Electrical harmonics — multiples of the line frequency; transformers and rectified supplies do this |
| A few hundred Hz to a few kHz (whine) | Switching supplies, LED/dimmer drivers, coil whine, early bearing failure |
The electrical hum: 50/60 Hz and its harmonics
A steady, unchanging hum at 50 or 60 Hz — often with buzzier harmonics stacked at 100/120 and 180 Hz — is the signature of the electrical system. The usual suspects are a ground loop (common when audio gear shares power with other equipment), a transformer or wall-wart under load, a dimmer switch chopping the waveform, or general powerline noise. If unplugging one device or flipping one breaker kills it, you've found your circuit.
The mechanical rumble: under 150 Hz
A low drone that doesn't sit at a clean 50/60 Hz — and that often changes when equipment cycles on and off — is mechanical. Furnace and AC blowers, bathroom and range fans, well pumps, aquarium pumps, and refrigerator compressors are the everyday causes. If the sound starts and stops with an appliance's cycle, that's your answer.
Step 3: The part a graph can't do — isolate it and listen
A spectrum analyzer can show you the frequency as a bright band on a graph. That's step one. But a picture of a sound doesn't help you walk through the house and find which wall, which outlet, which appliance.
What actually locates a source is hearing only that frequency. Set a band-pass filter tightly around the offending band and everything else — the TV, the traffic, the fridge — drops away, leaving just the hum. Now the sound becomes a game of hot-and-cold: it gets louder as you get closer. Walk toward it, and it leads you straight to the outlet, the transformer, or the fan that's doing it.
Where BandPass helps
BandPass is a free Android app that does exactly this. A live spectrogram shows the mystery noise as a bright band so you can read its frequency, and then you touch-drag a band-pass filter around it and hear only that frequency in real time — the step that turns a graph into a direction finder. Isolate the hum, walk the room, and follow it to the source. No ads, no subscription.
Get BandPass free on Google Play →A quick field method
- Watch the spectrogram while the noise plays. A steady hum shows as a bright, unmoving horizontal line — note where it sits.
- Check for harmonics. A stack of evenly spaced bands (60, 120, 180…) says electrical; a single low band that wanders says mechanical.
- Isolate that band with the filter so you hear only the hum.
- Walk the space. Louder = warmer. Let it pull you to the source.
- Confirm by killing power. Unplug or breaker off the suspect; if the band vanishes, you're done.
The bottom line
A mystery noise feels random until you give it a number. Low and wandering is mechanical; a clean 50/60 Hz with harmonics is electrical; a high whine is something fast or failing. Read the frequency to know what it is, then isolate it and let your ears lead you to where it is. That two-step — see it, then hear only it — is how you finally kill the buzz that's been driving you up the wall.
For the ear half of that job, BandPass is free, private (audio never leaves your phone), and built for exactly this kind of hunt.