yeah if you jump in big rainbow tunnel you will hear that sound too

So there was a secret...
I had to run around and confirm some things, which in the end, it does.

There are 5 spots you'll need to trigger, there will be a sound cue if you manage to trigger it.
1. Near where you spawn, another room where it has a big tree in it, if you look up and pay attention, there will be a baby doll encased inside a transparent blue glass box, shoot it, then pick the doll up, this will trigger it.
2. Same near the spawn, go to the opposite of where the tree room, enter the door, then go left, after that go left again until you meet a very tall ladder climb, climb it to reach the top and touch the Blue Skull Graffiti, this will trigger it.
3. At spawn, apparently there's a trigger in the vent ceiling, you will need a way to get to it, which there are two ways, First find the Rainbow Tunnel Chasm then just straight jump into it, you will be teleported in the vents of the spawn, or Second the closest is where you find a room that spins , the room in question is between a weird conjoined houses and a practical room with a secret bookshelf entrance, falling from the vents will trigger it.
4. As said above from OP, there's a small parkour room you can find, on the largest block space with one elevated one, behind there's a hidden switch, interact with it to trigger it.
5. Find a vegetation hallway where there's a fake wall, enter the said fake wall, which then will trigger it.

After you successfully trigger all five of them, a text and a sound will play that you managed to reveal a secret, which at this point, I still don't know where the secret is revealed.

Not part of the secret but there's a cool dancing charple out of bounds in the concrete / greyscale place w/ boneworks music located NW of spawn (if you consider the wall with the most test tubes in spawn as North, he's to the right of the cylindrical room inside the leftover space

wheres the secret?

引用自 Azure：

Not part of the secret but there's a cool dancing charple out of bounds in the concrete / greyscale place w/ boneworks music located NW of spawn (if you consider the wall with the most test tubes in spawn as North, he's to the right of the cylindrical room inside the leftover space

This is an interesting find, surprisingly you can enter the area normally through a fake wall (Before you enter the huge cylinder room with a cylinder pool in the middle and doors all around)

the fake wall is actually a bug and i was too lazy to fix it. so i put a dancing charple there