I had what sounds like the exact same thing on my 2013 DS. It was JUST infrequent enough that at first the LBS didn't even believe me. So I rode it around with one of their guys riding along side me...it did it once and he said I shifted wrong....then we were almost back to the store and it did it again while we were just pedaling leisurely. "Did you here that?!?" I asked. "yea...something's wrong".
Long story short, it was a wonky cassette, replaced under warranty that same day. Has never happened again since.