The drone of an airplane flying overhead. I couldn't rightly say why, but I remember being young and listening to a passenger jet flying across the sky. It was a spring day, maybe early summer. When I heard it I remembered being on a walk with my class coming back from some field trip.
Even now, if there isn't much going on and I hear a single airplane flying I stop and remember remembering that.
I don't remember what significance it had or why it has that effect now. It's lost somewhere in my own history.