{updated} C-130 : RAF Hercules

Having said that the sound of these were very familiar to me, in my youth ...
I was in a window-less lecture theatre in 1981 when I heard the distinctive hum, I mentioned to the guy sitting next to me that I could hear Herkis. After we left the theatre, the birds flew back down the valley in front of us ... proof of my hearing them earlier was another student pointing them out to someone and saying "there they are, coming back again, I saw them about 30 minutes ago ..."