Flutterby™! : Background Hum & Electrical Transmission

Mashable: YouTuber breaks down how to timestamp a call using just the background hum

The video, from YouTuber Tom Scott, features University of York research associate in the Department of Language and Linguistic Science, Amelia Gully, using a technique called Electrical Network Frequency analysis (ENF) to match background hum recordings with data from the UK's power grid (it's broken down in more detail in the video, but it essentially involves writing code that matches the recorded hum of the call with the publicly available grid logs).

The video: Tom Scott: The hidden background noise that can catch criminals