Grinding/knocking noise

My car all of a sudden started making a very loud grinding sound when I accelerate but it does not do it every time. When it makes the noise it last only about ten seconds or so then stops. If i let off the gas pedal the sound immediately stops. I took it to a mechanic who said it sounded like brakes and changed them out. He said he never heard the noise I was talking about. I picked the car up and of course it is still making the same terrible sound. I am taking it back to him but would like some idea of what to tell him to check this time. Any ideas?