Whatever this was 5;45 AMIt was loud and sounded like a jet flying low/fast and sounded like it circled back for a sec which if it was landing at the airport it would change direction alittle to line up with the runway.It was startling to hear this in Greeley.I have seen the fighter jets and this was big whatever it was. I think it was a jet by the engine sound but no lights? Was higher than treetop but still I would say below cloud cover.It may bave been an 18 wheeler cause of our proximity to HWY 85. But the sound was different.I am puzzled by this.