It's the sound of freedom. I love the sound of a jet turbine. Sprinkle a little bit of Whop-whop-whop from a chopper and I'm in the zone.
I went to elementary school at Shaw AFB SC in the 70's where the noise of flight's of F-4's taking off would cause the teacher to pause until the volume decreased. In my mind that's the sound of freedom. More of a median bell curve effect; low, increasing to high, decreasing to low.
If OP experienced what I think he experienced it's more like a sudden roar/earthquake style that hits with a bang then fades. Not the norm which makes you think WTF was that.
Here's a sprinkle of fan blades: