I think that some of the guns used by farmers also make that whistling sound that you are referring to.

I now live and ride in a mostly agricultural area. I moved here from the city. The first time I was riding out a country road and I heard the "bird gun" I thought someone was shooting at me--having lived in the city for many years, gunshots were an almost everyday occurrence.

I rode my bike into a ditch and dove for cover.

After realizing what was going on, I got up and continued my ride...albeit covered in dirty and mud from the ditch.

I am just glad that no one saw me pulling that maneuver--I would have been pretty embarrased.