Here is the same recording, played at 3x speed, this affects the pitch of the recording and makes the sound stand out a lot more. It starts at low volume from the begining of the recording and, at this speed, stops at around 8.5s. Change the display to spectrogram and you can clearly see it... Here is the original recording; http://freesound.org/people/AlienXXX/sounds/348515/#description Recorded from my bedroom at around 9-10pm. The windown was open. Does not sound like a car/motorbike/train/airplane. There is nothing in the room that would make any sound (refrigerator, air-conditioning, fan, etc). I don't know what it is.
神秘的不和諧humming_HP1kHz_x3speed.wav
聲道：2
類型：wav
dissonant、humming、mysterious、night、strange、recording、fast-forward、pitched-up、unknown
不搭調、低唱、神秘、晚、奇怪、記錄、快進、金字了、未知
資料來源：freesound ( https://freesound.org )