at 700 - 800 rpm prop makes a ringing/howling noise is this a problem?

Did you just get the prop redone? If the shop rounded over your edges it will cause it to whine. I had to take mine back in and have then grind the edge down flat, so it caused a sharp edge. Then the whine will go away. I don't think it hurts anything, it just is annoying.

harmonic prop , my 644 does it at idle speed and I love it.