I recently bought an 04 trailblazer and I have been hearing a ping noise whenever you first accelerate from a complete stop or from driving very slow. Sometimes it seems to be right under the front seats and other times at the rear of the vehicle. I have gotten under the thing and I cannot determine what it is. The only way I can replicate a similar sound is if I tap the drive shaft with my wedding ring. I see nothing that could be hitting the drive shaft. I am at a loss here. Anyone have any ideas?

Without checking this onsite, almost impossible to diagnose.

I finally took this to the shop and they said that there was something loose in the drive shaft and it needs to be replaced. Is this common?

Could it be a loose heat sheld, or heat exchange coming from the catalytic converters.

Loose in the driveshaft? What exactly is this?

Go to a local parts store like PepBoys, and put it on a rack.
1) Have them mussel and move the drive shaft, to check your
universal joints.

When universal joints go bad, it willl make more like a clunking type noise everytime you put it in drive.