Noises, smells and leaks: what your car is telling you

When a new sound, smell or puddle turns up, note when it appears, where it seems to come from and whether anything else changes at the same time. A short phone video, a photo of a leak or the exact warning shown on the dash can help when the symptom disappears before the car reaches the workshop.

This guide gives likely directions, not a diagnosis. Sounds travel through a car, fluids change colour with age and one smell can have several causes. If braking, steering, temperature or oil pressure changes, stop safely first and ask for advice before driving further.

Car noises and what they usually suggest

A squeal on braking often comes from brake pads, their wear indicator or surface contamination. It should be checked soon. Grinding while braking is more urgent because it can mean the friction material is gone and metal parts are contacting. Stop safely and arrange advice.

Clicking when turning commonly points to a CV joint, especially when it is rhythmic and louder under power. A hum or growl that rises with speed can come from a wheel bearing or a tyre. If it changes as the car corners, load shifting across a bearing is one clue, but inspection is needed to separate the causes.

Knocking from the engine needs prompt attention. If it is new, heavy, rises with engine speed or appears with an oil warning, stop the engine. A whine may come from a belt-driven component, power steering on some cars, a transmission, differential or wheel bearing. Note whether it follows engine speed, road speed, steering input or gear choice.

Smells that should not be ignored

A sweet smell usually suggests coolant. Look for a low coolant warning, coloured residue or rising temperature, but do not open a hot cooling system. Burning oil can smell sharp or smoky when oil reaches a hot engine or exhaust surface. Check the oil level safely and book the leak before it worsens.

Burning rubber or clutch may follow a slipping belt, a hose touching something hot, a dragging brake or a clutch overheating. Stop if there is smoke, loss of drive or a wheel that feels unusually hot. A rotten egg smell can relate to the exhaust treatment system or a battery problem. Keep clear of a battery that is hot, swollen or venting.

A petrol smell can mean fuel is escaping. Stop safely, switch off and keep away from flames or sparks. Do not keep driving to see if it clears. A musty air-conditioning smell usually comes from moisture and growth around the evaporator or cabin filter. It is normally a book-in-soon item unless fumes, smoke or an electrical smell are also present.

Leak colours under the car

Place clean cardboard under the parked car if it is safe to do so. The position, colour, feel and smell provide useful clues. Do not crawl under a car supported only by a jack, and avoid touching an unknown fluid with bare skin.

• Clear water after the air conditioning has run is usually normal condensate. It should feel like water and have no strong smell.
• Green, pink or orange fluid is usually coolant. Colour varies, so a sweet smell and the location near the cooling system are additional clues.
• Brown or black oily fluid is often engine oil. Old transmission or differential oil can also darken, so location matters.
• Red or pink oily fluid can be automatic transmission or power steering fluid on many cars. Some cars use other colours or electric steering.
• Clear-to-yellow and slippery fluid may be brake fluid. Do not drive. Brake fluid can damage paint, so avoid spreading it and arrange safe advice.
• Blue fluid is usually windscreen washer fluid. Check the washer bottle and hoses, but remember that some coolants can also appear blue.

Fluid colour is not proof by itself. Take a photo, note the position under the car and check fluid levels only where the owner’s manual gives a safe method.

Stop now or book in soon?

Stop now for an oil-pressure warning, rising temperature or steam, a brake warning with changed pedal feel, a strong petrol smell, smoke, heavy engine knocking, grinding brakes, sudden steering change, a rapidly deflating tyre or clear-to-yellow slippery fluid that may be brake fluid. Move out of traffic if it is safe, switch off when appropriate and ask for advice.

Book in soon for a new squeal, clicking on turns, a speed-related hum, a small oil or coolant seep with stable levels, a musty air-conditioning smell, an intermittent warning or a minor vibration that has not changed control of the car. Monitor it and stop if it becomes louder, faster, hotter or begins affecting braking, steering or warning lights.

When you call, describe the symptom in ordinary words. Say whether the car was cold or hot, moving or idling, braking or turning, and whether the noise follows engine speed or road speed. Our diagnostics page explains how those clues are turned into tests. You can also use the car care guide for routine checks that may catch a problem early.

Questions about noises, smells and leaks

Is clear water under the car normal?

Usually, if it appears after the air conditioning has been running. Condensation drains as clear water and is normal. Check that it has no oily feel, strong smell or colour before assuming that is the source.

What should I do if I smell petrol?

Stop safely, switch the engine off and keep away from flames or sparks. A persistent petrol smell can mean fuel is escaping, so do not keep driving to see whether it clears. Arrange advice and safe transport for the car.

Does a new grinding noise mean I should stop?

Usually, yes. Grinding can mean metal parts are contacting where a lining or bearing surface should separate them. Stop safely and get advice, especially if braking, steering or wheel movement has changed.

Can the colour of a leak identify it for certain?

No. Colour is a useful clue, but fluids vary and dirt changes their appearance. Note the colour, feel, smell and position, take a photo if safe, and have the car checked before relying on the colour alone.