I'm German currently in Canada and I heard the Hum apparently quite often in the last few days, never in Germany though. I've been on a very remote ranch somewhere 3 hours from Kamloops, BC and at first, I thought it was indeed a diesel engine starting up. Thing is, there was no neighbor close to that huge property. Then I heard it also 500m further away, same noise, same volume and also the same pattern. It's just for a short moment, 3-5 single tones in a row.
Then I drove with an ATV to some lake in the middle of nowhere and heard it again. I asked the other person I was with and she just said "well, it's someone starting a diesel engine". And I thought again: Who does start a diesel engine in the middle of nowhere. I thought it might be an animal or so, but could never tell for sure.
Edit: The interesting aspect is that I heard the Hum probably 3-4 times. 2x I was alone and in both other occasions, two different people heard the same sound. So I guess it was not just in my ears.
Your descriptions of a 'diesel engine starting' are different than the description in the wiki article of a diesel engine running. A running diesel engine would make a droning hum, while a starting one would click or tick and thump faster and faster into a hum.
That start-up noise is very similar to the sound of a grouse, a common bird in rural areas. A friend and I were hiking for a few days a few years ago, and we felt completely isolated and hadn't heard human noise for days... except that someone kept starting an engine a few hundred yards away! We knew it must not be people, but were baffled.
When we finally got back we asked his family members if they knew the source of those noises, and they showed us a youtube clip about grouse. Surprisingly noisy little things. I would seek a video for you, but I am on a restrictive telephone.
Well, but isn't starting a diesel engine something to be expected in rural Canada?
I mean, generators, trucks, etc?
Now see, in distances, it appears that lower frequency sound waves travel better. For example, take someone that's hearing music far away from you, the higher frequencies sound distinctively dampened.
So I wouldn't be surprised if a diesel engine could be heard a very long distance away in a quiet region
Then I drove with an ATV to some lake in the middle of nowhere and heard it again. I asked the other person I was with and she just said "well, it's someone starting a diesel engine". And I thought again: Who does start a diesel engine in the middle of nowhere. I thought it might be an animal or so, but could never tell for sure.
Edit: The interesting aspect is that I heard the Hum probably 3-4 times. 2x I was alone and in both other occasions, two different people heard the same sound. So I guess it was not just in my ears.