While on vacation in Oregon the last time I brought out my quad I noticed almost immediately a scratching/grinding sound when the rotors started. When it is off by turning the propeller I can recreate the issue. The sound is coming from one motor. I have not had the time yet to totally take it apart and investigate (....possible it's a piece of sand between motor and body, could be sand/dirt IN the motor though I didn't have any hard landings) but my husband seems to think a bearing may be messed up in the motor due possibly to flying with some wounded propellers and some insane wind conditions. Hell I don't know if the motors have bearings LOL anyway, I am accepting I may have to replace a motor. Should I replace all four or just the one? I've replaced the shell and done a brain transplant before but never gotten down to the nitty gritty. Husband works on slot machines so any soldering I can throw his way but I am concerned of losing balance if I replace only the one motor. Anyone experience this or have an opinion on it?