void NormalNoise()
{
    counter += Time.deltaTime * speed;
    Vector3[] vertices = mesh.vertices;

    for (int i = 0; i < vertices.Length; i++)
    {
        float xCord = (vertices[i].x + counter) * scale;
        float yCord = (vertices[i].z + counter) * scale;

        float noise = Mathf.PerlinNoise(xCord, yCord);

        vertices[i] = noise * height * mesh.normals[i] + originalPos[i];
    }

    mesh.vertices = vertices;
}

vertices[i] = noise * height * mesh.normals[i] + originalPos[i];

vertices[i] = noise * height * originalNormals[i] + originalPos[i];

private Mesh mesh;

private void Start()
{
    mesh = GetComponent<MeshFilter>().mesh;
}

private Mesh mesh;
Vector3[] originalNormals;

private void Start()
{
    mesh = GetComponent<MeshFilter>().mesh;
    originalNormals = mesh.normals;
}