The Noise Only Happens at a Certain Speed or Load: Decision Tree

Why this matters

A noise that is absent at idle and only shows up under a specific speed or a specific load is one of the most useful diagnostic facts you can get, because it splits the fault tree in half almost immediately. Speed-dependence and load-dependence point at different fault families, and the difference between the two is the whole diagnosis in most of these calls. This tree uses that one dependency as the primary branch, ahead of the sound's character, because it is the single most reliable discriminator when it is present.

Start here: reproduce it under control, and note which variable it tracks

Run the unit and deliberately vary speed and load separately if you can (throttle up without load, then load it at a fixed speed). You are trying to find out which one variable, if either, makes the noise appear or worsen.

If it appears only above a certain speed, regardless of load: go to the speed-dependent branch. If it appears only under load, regardless of speed: go to the load-dependent branch. If it needs both together: note that explicitly and treat it as load-dependent first, since load is usually the harder-working variable, then revisit speed if the load branch does not resolve it.

If it's speed-dependent

The noise tracks how fast something is turning, not how hard it is working.

• If it appears only within a narrow speed band and clears above and below it: suspect resonance. A structure or bracket is resonating at that specific speed. Press firmly on nearby panels, brackets, or lines while the unit is at that speed; if the noise visibly drops while you press, you have confirmed resonance, not a failing part. See the companion Reference on telling resonance from a genuine fault.
• If it appears above a certain speed and gets steadily worse the faster it goes: suspect imbalance or a developing bearing fault. Imbalance scales smoothly with speed; a bearing fault often adds a rougher, grittier texture on top of the smooth scaling.
• If it appears only at very low speed and clears once running speed is reached: suspect the unit passing through a resonant or critical speed on its way up, which is often benign if brief and if it fully clears at running speed. If it does not fully clear, or the shake at that low speed feels rough rather than a clean amplified shake, treat it as a genuine developing fault instead of a pass-through resonance.

If it's load-dependent

The noise tracks how hard the unit is working, not how fast it turns.

• If it appears only under heavy load and the unit sounds fine unloaded or lightly loaded: suspect a component nearing the edge of its capacity: a motor, a pump, or a drive component being asked for more than it comfortably delivers right now. This can mean a developing internal fault, or it can mean the demand on the system has grown (added fixtures, added duct runs, a restriction that developed) beyond what the component was sized for; check for a system change before condemning the component itself.
• If it appears only under load and is a knock or a clunk rather than a smooth increase in volume: suspect play or backlash in a coupling, a gear, or a mounting that only takes up slack once real force is applied. At idle there is no load to press the slack out, so the noise disappears.
• If it appears only under load and the noise is accompanied by a smell or heat: treat it as a component being overdriven or a resistive connection heating under current draw. Localized heat at an electrical connection under load is a shock and fire risk; de-energize and verify dead before further inspection.

If it needs both speed and load together

If the noise only shows up at high speed under load, and neither high speed alone nor high load alone reproduces it, suspect the combined condition is pushing a marginal component (a bearing, a belt, a coupling) past its threshold only when both stresses stack. Isolate by testing high speed with a reduced load and moderate speed with full load; whichever partial test gets closer to reproducing the noise tells you which variable is doing more of the work.

Recap

1. Reproduce the condition and vary speed and load separately to find which one drives the noise.
2. Speed-dependent, narrow band, clears above and below: check for resonance with the press test first.
3. Speed-dependent, worsens steadily with speed: suspect imbalance or a developing bearing fault.
4. Load-dependent, smooth: suspect a component near its capacity, or check for a system demand change.
5. Load-dependent, a knock or clunk: suspect play or backlash taking up slack under force.
6. Both together: test each variable partially to see which contributes more.

References

• ISO 13373-1, Condition monitoring and diagnostics of machines, Vibration condition monitoring, general procedures.
• Trade-standard practice for load and speed-dependent fault isolation on rotating and cycling equipment.
• See related: The Difference Between a Resonance and a Genuine Mechanical Fault; A Whine, a Rattle, or a Hum: Which Points Where.