Clicking Brooks Cambium C-17

My recently purchased Cambium C-17 started clicking when I return to the saddle after standing on the pedals. It is audible, but not loud. It's tactile as well so I feel it in my bottom. I've checked the rivet bolts as well as the mounting on the seat post and all are tight. The interesting thing is that it does not always occur. Some rides are click-free despite my trying to make it happen.

Any ideas? Should I be thinking about returning it as defective?