I can’t answer your question but I think I hear the same as you? Sounds like a prop, really low pitch drone? I have been trying to find out about this aircraft for ages. Came to the conclusion it was some kind of mail plane.
Don’t here it so much now as I have moved to a different part of Halifax but always passed over my old house at midnight give or take 5 mins.
Yes, some nights i hear a prop, but others its a jet.