When you do your regular servicing, have your mech check if your absorbers are leaking. If they are not, it could be that the bump stop(s) have "rotted" and broke apart.
I had a problem with sound from my fronts when going over humps and when I reach the bottom of slopes, but my workshop said the absorbers were not leaking. I changed to a full set of Bilsteins, but during the installation I saw the bump stops on the stock absorbers ... or rather what was left of them
I'm not sure if it is more cost-efficient to just change new absorbers stock-for-stock or have your workshop cut out new bump stops for your existing absorbers if they are not leaking though ... but I believe labour will be either same or close to in both cases.
Hope that helps.