What this noise usually means
What this noise usually means.
A hissing sound from the engine bay typically means a leak — coolant, vacuum, or boost pressure. The timing of the hiss helps pinpoint which one.
Common causes
Common causes.
- Coolant leakA pinhole in a hose or expansion cap; often hisses after switching off.
- Vacuum hose leakCracked vacuum line creates a constant hiss while the engine runs and may trigger the engine warning light.
- Turbo boost leakLoose intercooler pipe causing a hiss under acceleration with reduced power.
- Brake servo leakHiss with the brake pedal pressed; reduced pedal feel.
How serious is it?
How serious is it?
Medium-high if coolant is involved. Overheating can damage head gaskets quickly.
Can I keep driving?Stop driving if the temperature gauge climbs or coolant is visibly low. Otherwise drive cautiously to a garage and avoid heavy throttle.
Typical UK repair cost
Typical UK repair cost.
£40–£420. Hose or vacuum line: £40–£180. Coolant leak repair: £60–£280. Boost system repair: £180–£420.
Prices are guides only. Individual quotes will vary by make, model, parts and labour rates.
When to contact a garage
When to contact a garage.
Within the week, sooner if your dashboard shows any temperature or pressure warning.
AutoEar Diagnostic Engine
Use AutoEar to diagnose this sound.
Record the noise, answer a few quick questions and AutoEar will return a likely diagnosis, safety verdict, urgency and UK repair range — then connect you with a trusted local garage.
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