My guess is it is the axle assembly with the joints. Mine failed at about 9k miles on the left side and felt like a CV joint failing, sounded like it was a fairly common issue with the early Volts, but not all of them. If it is just the axle maybe the splines in the shaft open up a bit and clunk? I don't know and didn't ask to see the part(s).
It was covered under powertrain warranty.
It was covered under powertrain warranty.