i have a rubicon 500 2002 and sometimes ,i mean sometimes when im at a stand still with my quad and i give it gas it makes a grunting or grinding sound,like the gears aren't catching.so i put it in neutral and then back in drive and give it gas and it then goes but sometimes it takes my a few times putting it in neutral and back in drive before it will go, without it making that noise.when it makes this noise my back tires dont spin.please help or some suggestions