The Noise That Changed: A Decision Tree
Why this matters
"It started making a new noise" is one of the most useful complaints a customer can give you, and one of the easiest to fumble. A noise that changed means something physical changed: a part loosened, a bearing dried out, a flow path opened, or a stress finally cracked something. The noise is the machine telling you where to look. Chase it methodically and you find the fault fast. Replace parts on a guess and you eat the labor twice.
Start here: characterize the noise before you touch anything
Get the noise into words before you open a panel. Four questions sort most cases:
- What kind? Grinding, squealing, rattling, knocking, humming, hissing, clicking, gurgling.
- When? At startup, only under load, only at shutdown, continuous, or intermittent.
- How fast? Does it track the speed of a spinning part (a motor, a blower, a pump) or the cycle of the equipment (on/off)?
- Did anything change recently? A repair, a part swap, a power event, a move.
A noise that rises and falls with shaft speed is almost always rotating: bearing, fan, pulley, coupling, impeller. A noise that fires once per cycle is usually valves, relays, expansion, or hydraulics. Write the answers down. They split the tree.
If it is grinding or squealing
This points at a rotating part with a friction problem. If the pitch tracks motor or fan speed, suspect a dry or failing bearing, a rubbing fan blade, or a slipping belt. Kill power, spin the part by hand, and feel for roughness, drag, or play. A bearing with grit feels gritty; a worn one wobbles.
If squealing happens only at startup and fades, suspect a belt that slips until it warms and grips, or a bearing that quiets once lubricant spreads. If grinding is constant and harsh, do not keep running it: metal-on-metal removes material every second.
If it is rattling or buzzing
Start with the cheap causes. If the rattle changes when you press on a panel or cover, it is loose sheet metal or hardware, not an internal fault. Tighten fasteners and add isolation where metal touches metal.
If it buzzes in time with the electrical cycle (a steady hum at line frequency), suspect a relay, contactor, solenoid, or transformer with worn contacts or a loose laminated core. If the buzz comes from a fan that also vibrates, suspect a blade with built-up debris throwing it off balance, or a bent blade.
If it is knocking, banging, or thumping
These are impact noises, and impact means something is hitting something. If it knocks once per on/off cycle, look at expansion (ductwork or pipe popping as it heats), water hammer (a valve slamming flow to a stop), or a hard-starting compressor.
If it knocks rhythmically while running, suspect a loose rotating part, a cracked mount letting the assembly shift, or worn linkage with slop. If banging is violent and new, shut it down: a thrown blade or a failing coupling can destroy the housing and become a safety hazard.
If it is hissing, whistling, or gurgling
These are flow noises. If something hisses under pressure (air or refrigerant), you may have a leak; trace it with soap solution or an electronic detector per the system type. If it whistles, suspect restricted flow forced through a small gap: a clogged filter pulling air past its edges, a partly closed damper, a scaled orifice.
If it gurgles, suspect trapped air or two-phase flow in a liquid line: a low charge, an airlocked pump, a drain that lost its trap seal. Gurgle plus poor performance usually means the system is moving air where it should move liquid.
Confirm before you condemn
Once the tree points somewhere, prove it before you sell a part:
- Isolate. Stop suspected sub-systems one at a time and listen for the noise to vanish.
- Touch the path. A long screwdriver or a mechanic's stethoscope against bearing housings and valve bodies tells you which one is loudest. Loudest is usually guilty.
- Reproduce on demand. If you cannot make the noise happen, you cannot prove you fixed it. Find the condition that triggers it first.
A noise that "comes and goes" with temperature, load, or run-time is a clue, not a dead end. Note the condition, because that is the fault's fingerprint.
References
- Trade-standard preventive-maintenance and vibration-analysis practice
- OSHA machine-guarding and noise guidance (general industry)
- Manufacturer service documentation for the specific equipment class
- See related: Why Things Vibrate Loose; It Gets Worse As It Runs: A Decision Tree