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Two days in a row, after turning off engine, parked, this long tone at 1.1947 kHz appeared and then faded away: https://clyp.it/xedrwsuh I just happened to be recording and caught it on the 2nd day. I've heard it in the past, but rarely, and assumed it was another car's brakes, etc.

Then turned the engine back on, and two short whistles happened at similar frequencies, but jumping from lower to higher: https://clyp.it/jkycuejh

spectrogram of first whistles at 3 frequencies

spectrogram of second whistles at 2 frequencies

I couldn't tell where it was coming from. Sounded like it was coming from all directions or outside the car, but I heard it in two completely different locations on consecutive days after turning off the engine.

2016 Toyota Corolla LE

  • Is it letting you know you have left your lights on? – HandyHowie Sep 10 '18 at 7:15
  • @HandyHowie No, that's a more intentional-sounding alert – endolith Sep 10 '18 at 13:53
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Or, if you have vacuum control of things like heat/ac, headlights, etc. When the car is turned off the vacuum accumulator can pull in air through any small hole or leak in the vacuum system. It's usually high pitched and usually lasts for less than a minute.

How do I know? I used to have a 280Z with vacuum hvac controls and that was one of the clues that I needed to track down the leak(s) in the system. Usually the end of the tubing where it attached to the reservoir or the actuators.

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Sounds like acoustical feedback in the audio system. Might you have a device (e.g., phone) plugged into the "aux" input in your dashboard stereo system.

  • No, nothing plugged into the Aux. I think the stereo was off, but I'm not sure. – endolith Sep 10 '18 at 13:54
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One possible source of unusual sound after engine shutdown is air conditioning (A/C) refrigerant pressure equalization. Does this sound occur on hot days after running your A/C? On a hot day the pressure of refrigerant in the condenser inlet is 350psi or so. Pressure in the evaporator core is around 30 psi. When you shut the vehicle off there is refrigerant flow through the oriface tube / thermal expansion control valve for 30 seconds or so after vehicle shut down. And that flow can definitely create weird harmonics.

What is outside temperature when you notice the noise? Is A/C on? Is this possible source of that noise?

  • A/C was probably on. I'll pay more attention to this when it happens again – endolith Sep 10 '18 at 19:02

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