Usually it's the mobo damaged by overheating (integrated audio device).
If you plug in earphone does it shutter ?
If it's still under warranty, take it back and hopefully resolves the problem.
Maybe for the 1st repair they say it's a hard drive problem but then maybe it was a mobo problem and changed it for a used one that had audio problem....
Ya it stutters with headphones, and I know for sure this is my laptop they gave back because it has the same dents and such as before, I was hoping I didn't have to give it back to them... If there is this many problems shouldn't they replace it?