NoiseBootstrap.cs

using UnityEngine;
using Unity.Entities;
using Unity.Collections;
using Unity.Mathematics;
using Unity.Transforms;

public class NoiseBootstrap : MonoBehaviour
{
    // Position, MeshInstanceRenderer, Cubeを持つGameObjectEntity
    [SerializeField] GameObject prefab;

    void Start()
    {
        var em = World.Active.GetExistingManager<EntityManager>();

        // 地面一面にCubeを敷き詰める
        const int lap = 100;
        using (var entities = new NativeArray<Entity>(lap * lap, Allocator.Temp))
        {
            em.Instantiate(prefab, entities);
            for (int i = 1; i < entities.Length; i++)
            {
                var x = i / lap;
                var z = i % lap;
                em.SetComponentData(entities[i], new Position() { Value = new float3(x, 0, z) });
            }
        }
    }
}

NoiseSystem.cs

using UnityEngine;
using Unity.Entities;
using Unity.Collections;
using Unity.Jobs;
using Unity.Burst;
using Unity.Transforms;
using Unity.Mathematics;

public class NoiseSystem : JobComponentSystem
{
    protected override JobHandle OnUpdate(JobHandle inputDeps) => new Process()
    {
        offset = Time.timeSinceLevelLoad
    }.Schedule(this, inputDeps);

    [RequireComponentTag(typeof(Cube))]
    [BurstCompile]
    struct Process : IJobProcessComponentData<Position>
    {
        public float offset;
        const float height = 2;
        const float per = 128;
        const float size = 0.05f;

        public void Execute(ref Position data)
        {
            var x = data.Value.x * size + offset;
            var z = data.Value.z * size + offset;
            var pos = new float2(x, z);
            data.Value.y = noise.psrnoise(pos, new float2(per, per)) * height;
        }
    }
}