A familiar situation: the car has been knocking all week, you arrive at a workshop — and it goes silent. The mechanic shrugs, you drive off, and the noise returns at the first junction. A phone recording solves that, but only if it is made properly. A bad recording is worse than none: it sends the diagnosis in the wrong direction. Here is how to capture a sound you can actually work from.
Preparation before recording
Half the quality is decided before you press the button.
- Switch off the music, the climate control and the blower. The heater fan is the number one killer of recordings: its noise covers almost everything.
- Close the windows if you are recording inside the cabin, and conversely open the relevant window if you are catching a sound from a particular wheel.
- Take the case off the phone if it covers the microphone, and wipe the microphone mesh — dust in it kills the high frequencies.
- Find a quiet place: an empty yard, an underground car park, an industrial area at the weekend. City background eats weak sounds.
- Bring a helper if the sound only appears while driving. Recording at the wheel is a bad idea for obvious reasons.
Where to hold the phone
| Where the sound is | Where to point the microphone |
|---|---|
| Engine, accessories | Into the engine bay, 20–30 cm from the part, away from the belt |
| Suspension, wheel | Through the open window on the relevant side, or into the arch while parked |
| Brakes | Out of the window at wheel level, while braking at low speed |
| Exhaust | Behind the car, a metre from the tailpipe, not right against it |
| Interior rattle | Next to the suspected place, 20–30 cm away |
Holding the phone in your hand is better than putting it on the dash: the body of the car passes vibration straight into the microphone and adds a false hum. If you are recording under the bonnet, mind the safety — hands and phone well away from the belt, the fan and the hot manifold.
What to capture, by regime
One file per regime is the main rule. A sound is diagnosed not by itself but by how it reacts to conditions.
- Cold start. Start recording before you turn the key and hold it for 30 seconds afterwards. The most informative clip for an engine.
- Idle on a warm engine. Twenty to thirty seconds of steady running.
- Raising and dropping the revs smoothly. This shows what changes with engine speed.
- Driving over bumps. 20–30 km/h over joints and speed humps, window open. The checklist for those tests is in diagnosing a suspension by ear, and the catalogue of sounds is on the symptom page knocking over bumps.
- Acceleration and coasting. The same stretch under power and in neutral — the difference tells a mechanic more than any description.
Voice notes and what to avoid
At the start of each recording, say a short introduction out loud: which regime, what speed, which gear, where you hear the sound. “Cold start, minus five outside, stood for two nights” is ten seconds of speech that saves the mechanic ten minutes of questions. When the sound appears, say “there it is” — that marks the moment more precisely than describing it afterwards.
What not to do: trim the recording down to the sound itself — the context before and after matters; record with call-app noise suppression enabled; send the file through a messenger with heavy compression if you can send the original; layer music or commentary over it. And do not try to catch everything at once — three short clean clips by regime are more useful than one long, muddy file.
A note on the suspension: if the knock only appears on particular bumps, record that specific stretch of road rather than driving in general.
If fiddling with settings is not appealing, the recording can be made in the Pro-Stuk app: it prompts the regimes, asks about the conditions and, after matching them, shows the likely causes with percentages. The finished card with the recording is easy to show to a mechanic — the conversation starts straight at the point.
