Strange high frequency features using snoise

[YurySedyakin]

I’m using snoise to generate height map like this:

private static float H(float x, float z, float midLevel, float amplitude)
{
    const float seed = 777;
    const float frequency = 0.02f;
    return midLevel + amplitude * noise.snoise(frequency * float3(x, z, seed));
}

And as a result I’m getting rare strange high frequency artifacts. Can anybody explain those?
BTW, I tried 2D version of snoise and it doesn’t produce artifacts like this.