The family across the alley from us has three energetic children. They are not, or not all, too young to play on their swing set. When they do, the sound of metal rubbing on metal as they swing mimics a range of voices. Saturday I thought that I heard pigeons on our roof, then realized that it was the swing. Other times it has sounded like human sobbing, or like crows or seagulls.
I spent time enough on swings in my childhood--we, too, had a swing set in the back yard. I believe I remember the sound, but back then I didn't compare it to any others.