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I have a 2014 Electric Drive and sometimes when I start the car a air compressor like sound comes from the rear of the car. It only happens for a few seconds then stops. It also happens sometimes when the car is stopped at a red light, but not usually. Is there something wrong or is this a normal function. I'd really like to know. Thanks in advance.
 

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It's the air pump for the brake system. It's running to pressurize the hydraulic system that gives you assisted breaking.

The thumping you hear when first driving off is the ABS system test.

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It's the air pump for the brake system. It's running to pressurize the hydraulic system that gives you assisted breaking.
Or more correctly, it is a vacuum pump for the power brake assist diaphragm - it replaces the intake manifold vacuum that an IC engine produces and is used for the power brakes.
 
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