I've also often heard a distinctive whooshing howl on a descending note - duration ~5 seconds - over my home ~20nm west of KDEN. Not associated with climb-thrust (or other) engine noise, so I've assumed it was related to inbound aircraft.
Definitely an "aerodynamic" sound, rather than engine thrust, and at an altitude (~8-10,000 ft. AGL) much too high/far away to be the sound of the mechanical movement/drive systems themselves. So I've always assumed it was flaps or speedbrakes being pushed into the airflow.
I've never been able to spot the specific aircraft type visually. I kinda thought it might be 757 flaps, since they are known to be uniquely "powerful" in affecting airflow, and United flies a lot of them in here. But I'll bow to Tomahawk if he's linked the sight and sound to 'Buses. United, and especially Frontier, fly a lot of those, too.
HOWWWWWOOOoOoOooooouuuuuu....