It's the infamous "shock knock" basically theres a design flaw that has been thought to be all sorts of things with no real proof of what it is. Honda has said it's the collar length in the rear shocks but I, as well as many others, have replaced those collars with no changes. There's also a brace behind the rear seat floor that some say attribute some creaks and clunking but I've dyno matted that area and I still have the noises as well.

My suggestion is either dynamat the crap out of the rear of the car or live with it.