Driving a 2013 Prius "Clunker"?

My 2013 Prius with 25,000+ miles has developed a low “clunking” sound every time I stop and the rear of the car settles. It seems to have come on gradually and now it doesn't seem to get any worse. My dealer is unable to find the cause and there is nothing loose in the back end of the car. The sound reminds me of some old leaf springs needing help. This isn't an awful sound but it’s very irritating and I’m embarrassed to give anyone a ride. I've driven a friend’s 2010 and it makes no noise when stopping. I’m about to go back the dealer again and attempt to get a manager to take a ride and see if he can hear the sound. The mechanic I dealt with claimed that he tried out a new car off the lot and it made the same noise – implying that I should just quit whining. Has anyone had or heard about a similar noise?? Thanks for your help. JJ