I always assumed it was one of these in operation. Appears there's a lot of them.

Question is, what are they digging?

I think its a Bullroarer or maybe someone with a really loud sound system and keyboard that likes messing with people.

PS: I really don't think it's a bullroarer.

It's grader blades. From CBC:

On Friday afternoon city spokesperson Alisa Thompson said she had a simpler explanation — it was a city worker grinding down the blade on a grader.

"The grader blade needed to be straightened. Kind of gets ground down, and it makes a very strange noise. It's as simple as that," said Thompson.

"The video, the YouTube video, is actually right around the corner from the arena, and that's where our employee was doing the straightening." Thompson said the city will try to replicate the sound and prove it was the grader this afternoon, although she admits she likes the more creative explanations better.

"Like some kind of epic battle between a Kermode, and UFO, with sasquatch and maybe a whale thrown in there. That would be great for local tourism."

http://www.cbc.ca/news/canada/british-columbia/strange-sounds-in-terrace-b-c-spark-stranger-theories-1.1374238 [1]

Also, don't believe anything you read on the Daily Mail. They steal, embellish and sometimes just plain fabricate their articles specifically to garner clicks.

Is this the hum? That sound that is mostly heard in port towns across the globe. Supposedly it's the sound of heavy industries modulated over distances, usually around 35hertz and below according to studies but a lot of times the local government orders things like fan replacements or transformer repairs to stop the sound or at least assure the locals that it's been handled and the hum is still reported even after such measures.