1999 Chevy S-10 grinding noise

I heard this noise for the first time today, i was in a parking lot and i just started to pull away and i heard this loud metallic grinding noise. i put the truck in park and gave it some gas and there was no sound. the sound is loudest in 1st gear. lately the truck has had problems goin into gear. it wont leave 1st gear unless i take my foot off the gas for a second. any idea why it's having this problem and what that sound is??? again it's an automatic, the sound is only there when the vehicle is moving, truck has trouble leaving 1st gear and accelerating