The train whistle. I hear it several times a day and at night. Sometimes paying no attention to it. Sometimes though it breaks through and I really hear it. Mainly at night when everything else is quiet. Haunting yet at the same time comforting. Not many things are constant - trains are constant - the whistle in the night is always there. That's why it's so comforting. It's always going to be there.
Photo by Lucas Allison