Weird Siren

At about 11pm tonight (Tuesday), I was sitting in my home office when I heard a bizarre siren noise outside my window. I went out to the balcony and listened and it was definitely a siren, but I couldn't tell where it was coming from. It seemed to be a couple of kilometers away, but very loud and quite consistent for about 10 minutes.

After 10 minutes, the tone of the siren changed, as if an engine downshifted, and then it faded away quickly (it took about 20 seconds). For another 10 minutes, I could still hear it, but barely.

Did anyone else in Montreal hear this?