Quirks with generating sin tones on the fly

I'm generating sinus tones in Unity 5.6.3f1 using the following code:

 private double _factor = 2.0 * Math.PI / AudioSettings.outputSampleRate;
 public double Frequency = 440;
 public double Gain = 1;
 public int Location = 1;

 private void OnAudioFilterRead(float[] data, int channels)
 {
     _increment = Frequency * _factor;
     
     for (var i = 0; i < data.Length; i = i + channels)
     {
         _phase = _phase + _increment;
  
         float value = (float) (Gain * Math.Sin(_phase));
         
         if (Location == 2) // left & right
         {
             data[i] = value;     
             if (channels == 2) data[i + 1] = value;
         } else
             data[i + Location] = value;
     }
 }

Now I'm experiencing two major issues with this:

1. The generated sinus tone sounds not so smooth, as if I would use any online web based generator. Sounds more like triangle instead sinus. As lower the frequency as worst it seems to be.

2. When I change fast the gain (e.g. using the Unity slider from the script), changes in loudness are not smooth. There is some kind of scratching noise to hear.

What have I done wrong? What can I do to fix these issues? Any help is appreciated.

Thanks, Thomas