That is exactly what it sounded like. Interestingly, I’m also hearing sirens from miles away in the opposite direction heading in that direction. To be clear, we’ve lived here over ten years and I’ve only heard sirens once. Now twice.
If an array of vent points were opened along a line that is roughly perpendicular to your location, the sound could have been very loud.
To have gone on as long as you describe, a lot of $$$ worth of gas would have been lost, the contents of many miles of pipeline. Perhaps they got a report of a leak somewhere and vented a whole section of pipeline in order to avoid an explosion in a populated area.