Under Villages, in my case my own build, massive amounts of Zombies can spawn i think due to the raid function in hard (Hardcore) difficulty. The Zombies spawn in a cave random in a 50 Block radius and somtimes produce massive amount of sounds and lag. Im currenty in a Bamboo jungle so there is not mutch space where the zombies could spawn.

Steps to Reproduce:
1. Make a Wold in Hard/Hardcore
2. Spawn some Villagaers in a Biom were mutch space for mobs to spawn is occupied by blocks e.g. a Bamboo Jungle.                                                                                                                 3. Place some Beds and workstations So it gets detected as a village.
4. Wait a couple of nights.