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A normal, working car, nothing wrong with it, makes a very distinctive sound when you turn the ignition. I have a vague understanding of a distributor spinning around real quick and firing the spark plugs, but I don't really grasp what's making the sound.

Could you explain it?

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The best I can do is "friction" ;) +1 Great question. –  jmort253 Jan 30 '13 at 4:23
    
Are you talking about the sound while you're turning the key and the starter motor is running, or the normal sound while the engine is running? –  R.. Jan 30 '13 at 6:55
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up vote 4 down vote accepted

The main noise you hear is the starter motor. This is a high torque motor whose purpose is to turn the engine at a rate sufficient to allow it to run.

If you run a starter motor with no load it is still quite noisy, but as you will hear when starting your car, it tries to get the engine up to speed, fuel is pumped in, the spark plugs fire, and if all is well the engine spins up. Otherwise you have to try again.

Because the cylinders are still compressing a fuel and air mix, the starter motor does not simply accelerate, but instead sounds quite variable as it passes points of high and low compression.

The load is so high that car manuals recommend taking a break after every few tries to avoid overheating the motor, and it is very easy to quickly drain a battery using the starter.

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It should probably be mentioned that the pulsing sound of the starter motor comes from extreme variation in the load on the motor (and thus the rate at which it turns) due to compression in the engine. If you operate the starter with all the spark plugs taken out (and thus no compression), it sounds a lot different. –  R.. Jan 30 '13 at 17:29
    
Updated-Thanks. –  Rory Alsop Jan 30 '13 at 17:59
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