While goose hunting one morning in rural northeastern colorado an unusual sound was heard in the sky for approximately 5 seconds. the sound was heard by all four hunters (4 adult males). the sound was unlike anything any of us had ever heard and there was no apparent source of the sound. the sound came from the northwestern sky but there was no aircraft or object visible to any of us. it sounded like the high pitched whine of a jet airplane approaching combined with other sounds resembling very sudden and rapid air disturbances. the sound was very loud - like a jet approaching from about half a mile away. we all expected to see a jet, but none appeared. none of us could account for the secondary sound (the one resembling rapid air disturbance). we all looked at each other dumbfounded after the sound ended. although we never heard quite the same sound we did hear some similar high pitched whine sounds far in the distance to the west occasionally the rest of the morning. we did not hear any unusual sounds past around 11 am.