Could be something in need of lubrication, when's the last time you had it serviced? There's lots of parts that just need to be kept greased or they'll start squeaking. As the car hits bumps, all kinds of parts move in various ways. See if you can replicate the squeak by pushing down on various parts of the bumper on all sides of the car. It'll help you localize the noise, and help the guys in the shop (if you take it here) diagnose the problem faster.