Understanding Halo 5's legendary ending?

The Halo Rings with known locations prior to Halo 5 are Installation 00 (the Ark), Installation 03 (where the Composer is found in Halo 4), Installation 04 (Halo 1 & a replacement in Halo 3 that is also destroyed), Installation 05 (Halo 2 aka Delta Halo), and Installation 07 (quarantined by UNSC and Sanghelios forces in Hunters in the Dark) There are still three Halo Rings with unknown locations, any one of these could be the one Cortana took over.

She is connected to the Domain and has current access to human intelligence systems, so finding a forerunner facility isn't hard. Also, now that the Domain is active she can freely move between systems throughout the galaxy. She is not limited to one physical location at one time anymore.

It's pretty simple if you ask me.

If you remember the story the consoles tell in Halo 4, they show that the universe was once populated with both humans, Forerunners, and other species the same. Only the humans were somewhat locked in battle with the Forerunners against the Flood.

Humans had developed technology similar to what they have now in the Halo universe. But the Forerunners had to cleanse the universe of the Flood in order to be able to save it. So they collected the DNA of every species and put them in the same ships we see the Master Chief on at the end of Halo 2. And once the Halos were activated they wiped out everything including the Forerunners. Hence why in Halo 4 the Knights actually had human remnants in them from a human civilization from before.

So the ships dropped the DNA on their respective planets and civilizations evolved all over again. At the end when you see the new Halo ring you hear Cortana humming the same thing the Monitor from the first Halo hums when floating around. And of course Lasky's scene does parallel the first Halo.

From what I gather with everything Cortana said and how she talks. And the fact that she said she would wake the Chief in 10,000 years, can only mean Cortana is now recreating exactly what happened the first time around. Only this time instead of the Flood being the problem trying to consume the universe she believes humanity is. And it looks like she plans to cleanse the universe of all humanity and start fresh.

I also have a small theory which I'm sure is extremely not true but does make you wonder: Maybe the Monitor from the first Halo actually gained control of Cortana, and that's why she's humming the same song when you see the Halo ring at the end. But most likely its just paralleling the first Halo.

To understand the ending Halo 5 Guardians requires including not only the stories and characters from previous games, but also Greg Bear's Forerunner Saga trilogy. Halo: Primordium ends with a revived 343 Guilty Spark having come to the conclusion that the Librarian is still alive in 2557, so it takes over the USNC Rubicon, puts the crew to sleep, and disappears with the ship. This leads me to one simple conclusion, but also a second more speculative answer:

1) the humming we hear is indeed 343 Guilty Spark, he has found a Halo installation (possibly where the Librarian is hidden) and he is happily settling in and reactivating it. A teaser for Halo 6, or a future Halo 5.X update, extra campaign.

2) my more speculative answer is that we are witnessing Cortana arriving at the ring, but the humming is a huge clue that the projection we have been watching is either not Cortan; but 343 masquerading as Cortana; or in some fashion similar to the Gravemind mind games, 343 has succeeded in corrupting Cortana. A wild idea, but consider what monitors are: very advanced AIs. 343 always thought he was smarter and more responsible than humans. Cortana's proposal that all AIs join her to protect and police the galaxy by assuming the Mantle is identical to 343's mission on installation 04, just blown up to galaxy level.

All of the rings can be activated as well as fired, from the arc.

I believe the better question is: "Why is Cortana whistling the same tune that 343 Guilty Spark was? "