Noise on Reunion (Spoilers)

Possible newb question, and I am no newb to Halo, just don’t follow the lore like some do.

On this mission when you hear that noise Blue Team recognizes before activating each of the three consoles, Fred said “How many years has it been since we’ve heard that?” Where is it from or originate from? I do not recall such a “signal noise”.

It’s a signal that they used to communicate with each other “Oly Oly Oxen Free”
It’s their code.

When they were abducted at 6 years old and taken to Reach to train to be Spartans, John, Kelly, Fred, Sam, and Linda used to whistle that tune “Olly, Olly, Oxen Free” as a signal to communicate when they were running exercises. They also used to take a finger or two and do a quick swipe across their visors in the shape of a smile since, you know, you can’t shoot a grin through a helmet.

I was really glad they included that in the game. But I’m not sure how Cortana would have known about it since they hadn’t used it since they were kids. Unless Halsey took note of it and put it in the record or in her journal. I could totally see her reading her journal to Cortana during the early stages of Cortana’s development. I mean, she was planning on giving her to one of the Spartans, so she would need to know everything.