I would think that any sand/dirt normally vacuumed would simply slip through the pump's impeller with the water and keep going to the filter. Nothing gets to the electric motor itself since it's sealed and separated from the "wet-end" portion. The only thing I could think of is if some debris got wrapped around that impeller (like twigs or something). You can try and reach in there (with power OFF) to see if you notice any junk lodged in that impeller area, but even then if there was, I wouldn't think those type of items like grass, twigs, and stuff would sound like a grinding noise. It's possible there is some residual rust or junk stuck inside the "dry-end" motor section where your bearings/motor shaft spin if your pump sat all winter. Perhaps remove the back cover and blow-out that motor area with some compressed air. I would certainly keep an eye out to see if it progressively gets worse and consider contacting the manufacture since the motor should still be under warranty.
If I missed something, others will chime-in I'm sure with more suggestions. Good luck!