Sound Diagnostics
Engine Sound Diagnostics
Diagnose engine noises by rhythm, pitch, load and temperature. Learn the order math that ties a noise to the crank, the cam or an accessory, how to record a clean phone clip, and what to send with it.
When sound beats the scope
A scope reads electrical events. Some faults never reach a wire: a worn bearing, a slapping piston, a failing accessory bearing, a loose heat shield. They show up as sound and vibration first. A good recording narrows the source before you open anything.
The four properties of a noise
1. Rhythm — which part is turning it
Every rotating part makes noise at a rate tied to its speed. Expressed as orders (events per crank revolution):
| Source | Order | Rate at 800 RPM |
|---|---|---|
| Camshaft and valvetrain (lifters, phasers, cam lobes) | 0.5 (once per two crank revolutions) | 6.7 per second |
| Crankshaft, rods, pistons | 1 (once per revolution) | 13.3 per second |
| Firing pulses, 4-cylinder | 2 | 26.7 per second |
| Firing pulses, V8 | 4 | 53.3 per second |
| Belt-driven accessory | Pulley ratio × features per revolution | Depends on the accessory |
| Wheels, tires, wheel bearings | Road speed, not engine speed | Changes with vehicle speed only |
Rate (per second) = order × RPM / 60. If the rhythm doubles when RPM doubles, it's engine-driven. If it follows road speed in neutral coast-down, it isn't.
2. Pitch — what's making contact
High, sharp sounds come from light, hard contact (valvetrain, small clearances). Deep, heavy sounds come from heavy parts or large clearances (bearings, pistons).
3. Load response
| Response | Typical sources |
|---|---|
| Worse under load | Bearings, detonation, belt slip, exhaust leaks |
| Worse on load reversal (blip and release) | Rod bearings, drivetrain lash, loose mounts |
| Unchanged by load | Valvetrain, accessory bearings |
4. Temperature response
| Response | Typical sources |
|---|---|
| Cold only, fading as the engine warms | Piston slap, lifters bleeding down, chain slap until oil pressure builds |
| Worse when hot | Bearings (thinner oil), detonation |
| Always present | Mechanical wear |
What you'll need
- A phone. Record with noise suppression and automatic gain turned off if the recorder allows it; these features remove exactly the sounds you need. Record at 44.1 or 48 kHz, uncompressed or high quality.
- The engine RPM for every clip, from a scan tool, the tachometer, or a video that shows the tach.
- Optional: a mechanic's stethoscope to locate the source.
Step-by-step capture
- Reproduce the noise first. Find the RPM, temperature, and load where it appears.
- Secure the vehicle: parking brake on, transmission in Park or Neutral, wheels chocked.
- Place the phone about 60 cm (2 ft) from the suspected area. Prop it on the fender or strut tower — never on the running engine — so hand noise doesn't get into the clip.
- Record one condition per clip, 15–30 seconds each: idle, a steady RPM, a slow sweep, snap throttle.
- Note the conditions in the upload's text field, not by talking over the recording. Speech covers the frequencies you need.
- Record at least two positions (for example, front of engine and each side), keeping everything else the same.
Recording tips
- Stop fans, radios, compressors, and conversation nearby.
- Record indoors or out of the wind. Wind overloads phone microphones.
- Avoid Bluetooth microphones and noise-cancelling apps.
- For intermittent noises, record continuously until it happens, then trim.
Safety
- Never reach across a running engine: belts, fans, and hot exhaust cause most under-hood injuries.
- Never work under a vehicle that isn't supported on rated stands or drive-on ramps.
- For noises under the vehicle while running, place the phone before starting the engine, then stay out from underneath while it runs.
- Never handle a phone while driving. For road-test clips, a passenger records.
Feeding the analyzer
Upload clips in the Sound Analyzer with:
- Year, make, model, and engine.
- The RPM and condition for each clip.
- Where the phone was placed.
- When the noise happens (cold start, warm idle, under load, coasting).
Frequently asked questions
Can I diagnose engine noises from a phone recording?
For many faults, yes: bearing knocks, belt and accessory noises, exhaust leaks, and chain noise all have characteristic rhythms that survive a phone recording. Knowing the RPM of each clip is what makes the rhythm measurable.
Where should I place the phone?
About 60 cm (2 ft) from the suspected area, propped on the body, never on the running engine. Record the same condition from at least two positions; the source is usually loudest nearest to it.
How long should each clip be?
15–30 seconds per condition. Long enough to hold many repetitions of the noise, short enough to cover one steady condition.
What conditions should I capture?
Whatever produces the noise, plus: cold-start idle, warm idle, a steady 2,000 RPM, a slow sweep up to about 2,500 RPM, and snap throttle.