[2021] I'm terrified. Seriously. Not joking.

Yeah, I heard some sounds I don’t recognize. They seem to fit the mood, so my best guesses are:

• Some sounds were added recently that I hadn’t noticed.
  
• You have a mod running that you forgot about.
  
• You have something else running in the background making noise.
  
• You’re in multiplayer and someone’s using their microphone to prank you.
  
• Your computer is haunted.

Originally posted by Amanoob105: If it makes you feel any better, zombies can't spawn in underground tunnels you, the player, have made.
Once you understand this the sounds quickly go from unnerving to just annoying. Which is, mostly, an improvement.

To anyone wondering, this is true in practice, but only because a zombie trying to spawn at those (x,y) coordinates will spawn on the surface instead.