WARNING: The following contains spoilers for Star Wars: Darth Vader #11, by Greg Pak, Raffaele Ienco, Neeraj Menon, and Joe Caramanga, on sale now.
Lightsabers are the most powerful tool of any Jedi or Sith. While Force powers greatly vary between Force-sensitive users, Jedi or Sith can always train to master the art of the lightsaber. And arguably, the most important aspect of the lightsaber is the Kyber crystal. Not only do the crystals power the lightsaber, but they also play a role in the lightsaber's color. The Sith almost exclusively use red lightsabers, and Star Wars: Darth Vader #11 just showed how horrifying their creation process truly is.
After being put through grueling tests by the Emperor, Darth Vader proved his worth and earned a Wayfinder to Exegol. He claimed to no longer be Sidious's apprentice before wreaking havoc throughout Palpatine's base. After defeating an army of cultists and gazing upon Palpatine's armada of Star Destroyers, Vader and Ochi of Bestoon descended into a glowing chamber. Within the chamber, there is a mountain of blood-red Kyber crystals.
However, the color of the crystals is not what draws Ochi's attention. A deafening high-pitched whine echoed throughout the chamber as Ochi clutched his ears. He asked Vader what the sound was and Vader explained that the Kyber crystals must suffer to reach their maximum power. The Sith cultists are constantly running blades along the Kyber to keep it in a constant state of suffering, which causes the crystal to scream.
The scream that echoes from the mountain of Kyber crystals seems to have a strength of its own that gets stronger when Vader is present. As Vader stares at the mountain, the scream grows louder and blasts the cultists backward. Ochi's helmet flies from his head and his eyes begin to burn from their sockets. Vader exclaims in anguish which creates a Force vision. In his vision, Vader is told by Luke that he can destroy the Emperor and the Emperor has foreseen this ending. It's a moment that's an explicit inversion of Vader's conversation with Luke in Cloud City during Empire Strikes Back.
While it's not specifically stated that the Kyber crystals scream while placed in the hilt of a lightsaber, there is enough information to infer that red sabers would make some sound. A Sith has to bend the Kyber crystal to their will and make it bleed in order to create the red hue. Clearly, there aren't cultists constantly torturing each individual lightsaber, but it is possible that there is something within the Sith's lightsaber hilt that could cause the crystal to consistently bleed.
It's also fair to note that most lightsabers use a minimal amount of Kyber crystal. Most blades only receive one shard, whereas Palpatine's base had an entire mountain of Kyber to power his armada. It's possible that the bleeding Kyber crystals do make a sound within a lightsaber but it is too faint for the user's ears. Palpatine had infinitely more crystals than a single lightsaber would need, and it's possible that the crystals could feed off each other or that their sound is just amplified in their current location.
Regardless, this adds a little more detail into what makes up a lightsaber and the atrocities the Sith commit to gain their power. It was already known that Kyber crystals were living creatures that are forced to bleed by the Sith to actuate their full potential. Now, Darth Vader has shown that the crystals scream when they are bleeding as well.