clunking noise from hatchback

Vehicle Information: Model (Boxster, Boxster S,Cayman, Cayman S): Cayman S; Year: 2007; Engine (type, size, modifications): stock; Total Mileage: 7,000; Transmission: Manual;

there is a sharp "thunk" that appears to come from the hatchback whenever a sharp bump is encountered. Reading the last Panarama, I see two other members complained about this problem. Are you aware of a easy fix for this? The car is still under warranty, but I prefer not to have anyone else tamper with my little baby.