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Q: Loud pitchy noise

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For the past year my BMW X3 suv deals with this loud pitchy noise that comes up at times when I'm driving. The strange part about it is that no light shows up on the dashboard indicating where problem is coming from but as soon I turn the car off and I can literally hear the noise slowly diminish and go away(almost like air coming out) . When I start it back up the noise is gone.

My car has 15800 miles.
My car has an automatic transmission.

The BMW X3 and X5 SUVs have an air suspension option, which utilizes an air compressor (under the spare tire), and compressed air shocks in the rear suspension. Your symptoms suggest a leak in this system: the air shocks themselves, air lines, check valves, etc. Your would hear this "air escaping" noise after shutting off the engine. Engine and other noises probably cover up this noise while driving. I would recommend having a car is making a noise inspection completed by a mobile, professional mechanic, such as one from YourMechanic, to confirm this diagnosis, get an accurate assessment of damage and cost estimate for repairs.

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