Noises can be annoying or ominous

Though there are few sounds more heart-warming than the song of a healthy engine, a motor car can also produce a range of disturbing noises, some merely annoying, others frighteningly ominous.

Though there are few sounds more heart-warming than the song of a healthy engine, a motor car can also produce a range of disturbing noises, some merely annoying, others frighteningly ominous.

The annoying category includes an assortment of rattles, whistles and buzzes.

Rattles are caused by the vibration of loose objects. Those inside the passenger compartment or boot are usually easy to locate and eliminate. Rattles from the undercarriage or engine might mean something is about to fall off.

Whistles at high speeds are probably wind noise so look at door and window seals. A whistle from the engine at idling speed might indicate the beginnings of a manifold gasket leak or steam escaping past the radiator cap gasket because of overheating, and this falls in the category of the ominous. Buzzing, a noise similar to a trapped insect, is mostly caused by interior trim parts rubbing against each other. Get someone to prod or press on every likely spot while you drive. Silicone spray at the right place often helps.

With ominous noises, there's an alarming array classified broadly as engine noises, drive-train noises and suspension noises. We'll focus on engine noises today and look at the other two next week.

Engine Noises:

l The dreaded "bearing knock" must head the list. It's unmistakable: a deep thudding, like knuckles against a heavy piece of wood. It usually means a big-end bearing is on its way out, but could also be a loose gudgeon pin. It's sometimes difficult to distinguish between them without further testing. Either way it's going to be expensive. If it's a bearing, urgent attention is needed. When several bearings are at death's door, the knock becomes a "bearing rumble".

l The ping, like glass beads poured on a tin roof, occurs when the engine is working hard at lowish revs, indicating detonation.

l The squeal of a loose drive belt.

l The screech of a water pump, idler pulley or alternator bearing on its last legs.

l The tap from a loose tappet, or a hiccup in the valve-train, sounds like rapid tapping on the intake manifold with a screwdriver.

l The sizzle, like bacon frying, when you switch off the engine. Caused by fluid dripping on a hot exhaust pipe or manifold. Not to be taken lightly; I once saw a car reduced to a blackened hulk within minutes.

l The bang, like a gunshot, when an engine backfires because of overly retarded ignition timing, aggravated by an overly rich mixture.

Unburnt fuel ignites in the exhaust pipe with enough force to blow apart a less- than-robust exhaust system.

l The pop when an engine spits back through the intake manifold because of a sticking or leaking intake valve or to jumped valve timing causing the intake valve to be open when it should be closed. Also called "flashback". It has been the cause of many engine fires.

l The flapping of a drive belt coming apart.

l The roar from a blown gasket in the exhaust system. Sounding like a Boeing about to take off, it will not endear you to neighbours. Leaking exhaust gas is potentially lethal, and there are well-documented tales of mysterious loss of concentration and severe headaches in passengers.

It is sometimes difficult to pinpoint the exact source of an engine noise. A great help is an automotive stethoscope.

You get professional mechanic's stethoscopes, but you can make your own inexpensive version not far behind the high-tech stuff. Take a length of hose pipe, jam a metal rod, a welding rod, threaded bar, or even a curtain rod, into one end and use that as a probe with the open end held to your ear.

It will amplify the noise where you place the probe, and allow you to locate the source by a process of comparison.

Just be careful: it's easy to concentrate so hard you don't notice you are placing the probe where a spinning fan blade will hit it.