This could mean many things, innumerable possibilities, I will give you the most common, at low speed the blades will not wiggle unleveled as much as when they are accelerated, that grinding noise would, most likely, be your blades touching the sides or top of the protection shell or cover in your machine. Remove and level with those inexpemnsive kits and a grinder, also when you hit a rock, or in any case a root, while you are in use of your machine, this will slightly bend one side of the blade making it impossible to see by eye but when the blades spin, this will be very noticeable by the noise made. Good luck