Grind while accelorating

Recently I got a 1995 Pontiac Grand AM GT with a V6 engine, I don't know alot about cars and that's why I'm here. I had some engine work done and replaced the part that keeps tension on the serpentine belt because it was worn out and causing a horrible clicking when the car idled. The car runs smoothly now but lately there's been a bit of a grinding noise when I accelarate. It scared me a bit so I asked a friend, he suggested getting the oil changed. I did that and they said there was less then 1/2 quart of oil in the car and it was very dark black. Having the oil changed does seem to have quieted the grind a bit but not totally, and there is a chance that the car could be running louder with more power and drowning out the grinding sound. Like I said I'm not a big car person so if I sound crazy that's why.. Could you guys give me some info on what could be grinding while I accelarate? Thanks very much!