There are a few threads discussing this. For some it resolves in less than a year post surgery, for others it does not. In the former case I think it is attributed to the scar tissue not yet having closed the pericardial sac. In the latter, case, which I have had for almost 6 years now post surgery, it seems to be associated to an aneurysm repair (not the valve as my repair was valve sparing). None of the Surgeons or Cardiologists that I mentioned this to were familiar with this. The best explanation I was given was simply that the surgery altered my anatomy somewhat and so the sound is transmitted differently (though it is more than the sound that is transmitted as I feel the thump more than hear it).
I imagine that my aortic graft is simply closer to the sternum than my aorta was and presses up against it. When I take a deep breath, arch my back, or lean my head back (look up), it feels like someone is flicking my sternum in time with my heart beat. It was quite annoying, and actually somewhat painful the first couple months after surgery but with time I got used to it. There were a couple people for whom it was so strong that it was quite disturbing, but for most I think it is just annoying.