To check gear mesh...take a standard piece of notebook paper and cut a long strip...say 5 or 6 inches in length by say an inch wide. Double this paper over "lengthwise" so you end up with a 5 or 6 inch by 1/2 inch strip (folded).

Run this paper through your main gear and pinion. It should come out looking nice and straight with no left or right curve to it and also "accordion" like in appearance with NO HOLES or TEARS. If it is barely ruffled (like the potato chip) then your mesh is too lose. If it is severely ruffled and pulls to the left or right with actual tears in the paper...your mesh is too tight. Given your symptom, I think the later is not the case here.

Back to another possible issue. Have you checked the motor timing on your ESC? Sometimes when the motor timing is not set correctly or the ESC doesnt have an automatic timing...this can cause a loud "shrill" sound which could be confused as a "grinding" sound. If you check your gear mesh and it looks to be okay...this would be my next step in the troubleshooting process.

goodluck