This has just started in the past few days. About 90 seconds after I turn off the engine, it will start emitting what sounds like an almost perfect sine wave at a little over 1kHz. It lasts for another 90 seconds or so, and then quickly fades out.
Audio here: https://db.tt/KKJ4IFls
Because it doesn't last long, I haven't yet been able to pinpoint exactly where it's coming from, but it seems to be from the engine compartment.
As you can hear in the audio, it continued even when I took the key fully out.