But hang on, we're not done yet.
Even the SR-71 is still a supersonic plane, not a hypersonic plane. And thus, the air rushing into the engines of an SR-71 is still doing so at a subsonic speed. For a hypersonic plane, this is different. The air rushing into the engines of a hypersonic plane is itself supersonic, which is why they call such engines "scramjets". And when it comes to scramjets, a number of interesting strategies have been adopted to increase efficiency and prevent damage to the engines from the continuous influx of air moving at supersonic speed. You have to keep in mind that at that speed, air is harder than concrete ─ well, you already know what air can do at much lower speeds, given the damage caused by the storm that I mentioned higher up in this post, and that was just at air speeds of 120 km/h.
And so, one of the techniques the USAF has been experimenting with when it comes to hypersonic planes is called "pulse-detonation"
. What this does is that, instead of allowing a continuous combustion process and thus with a continuous exhaust jet, the air/fuel mixture is ignited in short bursts, resulting in a pulsed exhaust. And this generates a very characteristic sound, very dissimilar to other jet engines. It is also not specific or exclusive to hypersonic planes, because the infamous German V-1 used during World War II was essentially an unguided cruise missile that traveled at subsonic speeds ─ i.e. slower than the speed of sound ─ by way of a pulse-detonation jet engine, which gave it its characteristic rumbling sound, which does indeed resemble the sound of a helicopter somewhat.
Now, the USAF is certainly still testing experimental hypersonic planes that use pulse detonation, and given how fast those things are ─ hypersonic planes are usually intended either to carry a nuclear payload or as spy planes ─ they can cover a lot of ground in very little time. So that could easily have been it.