1996 GMC Jimmy loud grinding noise when I start the truck and hesitates

1996 GMC Jimmy mileage: 84,000. I have recently had problems with starting my 1996 GMC Jimmy. When I turn the ignition key sometimes it will turn over with no trouble and others will make a loud grinding noise and will sometimes hesitate to start. If I give it a little gas, it will help but the grinding noise continues. It sounds like when you leave the key turned for to long and the starter is still trying to start the engine but it is happening before the engine is even running. Once it is started there are no problems at all-knock on wood! This problem just started two days ago and would like to get it resolved before I get stuck somewhere.