I have had my P4 for about 7 months, and I am starting to hear a quiet and strange noise when I manually turn 3 of the four motors. It is not loud, but the three motors do not feel like they are moving completely smoothly. I have heard of this sort of thing happening before, but this does not seem to be as loud as I have seen in videos. It also flies completely fine. So should I send it in, or just ignore it?