Grinding Noise

I recently bought a 2000 Mustang GT Vert 5spd. After buying it i found out what i thought was tire noise was actually the rear differential, so i had that rebuilt and 3.73 gears installed. Before that i had some grinding noise which i thought was associated with the rear diff. I have the noise when i accelerate from a dead stop in 1st gear. It doesn't gind with any other shifts, i have noticed a grinding noise that is similar when i was coming to a stop to part this morning. I know that the noise i am describing is pretty general but any input would help, considering i just spent a bunch of money for labor to have the rear diff done i am hoping this won't be expensive.
 
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So the grinding could be from the axle shaft if its pitted and the bearing has worn. It could also be a bearing noise from the trans or any of a thousand other issues. Have a buddy either drive the car and you stand close by to listen or you drive and have them listen. You have to isolate the noise and identify the location or we are just throwing darts blindfolded.
 
some new info on the grinding noise. Seems to only do it when i'm at a stop and i accelerate at a low RPM. If i'm at or above around 1800 RPM it doesn't do it. It sounds similar to if you shift but don't get in gear all the way and it makes the grinding noise. But this grinding noise is as loud as that, think it may be coming from either lower transmission, back of transmission or back of engine, but its hard to narrow it down to exact location. The only other time it does it when i'm coasting and put it into 2nd and go, but it is still at a low RPM. Taking it in to a shop to get checked out on Saturday, but having an idea of what it is and know if it is safe to keep driving.