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Noise tracing is best done systematically, and the first thing to do is to decide if the noise is engine-speed or vehicle-speed related, and if it's audible while the car is stationary with the engine running. If you find that the noise comes from under the bonnet with the car stopped, you'll need a listening device.
Engine stethoscopes are available, but a section of garden hose works extremely well, and even a large screwdriver will do. Hold the one end to your best ear and bring the other end near to the part whose noise you want to sample. You'll have to be super careful, because this search is potentially dangerous since it's easy to touch hot or rotating parts while you're concentrating on that irritating sound.
The major noise-making parts in the engine are as follows:
1. Alternators often make whistling noises, which can normally be ignored if the warning light is off. A worn alternator bearing sounds very much like a water pump bearing, but your listening device will soon enable you to decide which bearing is at fault. A hum could be due to failed diodes, in which case the alternator will not charge properly.
2. Pinking, a tinkling noise when you accelerate in a high gear, stopping the moment you reduce the pressure on the accelerator, can have many causes. It may mean that the ignition timing is too far advanced, and this is the first thing that should be checked. If the noise doesn't go away when you retard the timing, then either the noise is not pinking, or the pinking is due to a number of other possible causes. These could include inferior petrol, carbon build-up, a broken advance spring in the distributor base, a faulty vacuum advance mechanism, spark plugs of the wrong heat range, an over-heating engine, or too lean a mixture.
3. Tappets make a clicking noise that increases in frequency as you raise the engine speed. If the valve clearances are set correctly and the noise persists, then further investigation by an expert is needed. Hydraulic tappets can be temperamental, making a noise that comes and goes, especially when you first start up. If the noise goes away as the engine warms up, it may be the wrong grade or even brand of oil, or it may be a small dirt particle in the oil. If the noise is present all the time, you should consult an expert.
4. Worn big-end bearings usually make a light knocking or pounding noise that is more noticeable when the engine is neither accelerating nor decelerating.
5. A squeaking noise can often be traced to a dry distributor cam or a fan belt..
6. Broken valve springs will make a clicking sound, and may cause hesitant idling.
7. Constant-velocity joints make a clicking sound when worn. The outer joints click under power on tight turns, and the inner joints will make a noise when you are going around a circle while easing the power on and off.
8. Universal joints, as used on rear-wheel driven cars and bakkies, usually last longer than CV joints because they don't have to work so hard. They also tell you when they are old by clonking as you accelerate just after you've snicked a car into gear.
9. Wheel bearings become noisy when worn out. It's usually possible to isolate a noisy front-wheel bearing by cornering with both front windows open. The worn bearing will get noisier if you put a load on it. If the noise worsens when you corner to the right then it's the left-hand bearing, and vice versa. Wheel bearings can also be checked by jacking the car up and spinning each wheel in turn by hand. The noisy bearing will feel rough. Rear-wheel bearing noise often sounds very much like the rear axle unit, but spinning it by hand should enable you to tell the difference.
10. Rear axle hum can be very annoying, but seldom requires an immediate fix. The only time to start worrying is when the noise increases. Just about any solid rod can be used as a passable noise detector.