Could be the fuel regeneration solenoid for the evaporative emissions system. Now, the next question, what the heck is that? With the new emissions laws, there is a charcoal canister in every car sold in the USA. In the Crossfire, this is mounted under the vehicle, directly under the fuel tank. This would place it near where you are hearing the noise.

The comments about only hearing it when the engine is warm and upon coming to a stop would make sense, since this is when there is the most fuel traveling in the system. Also, since the system is closed loop, fuel is always flowing back to the tank from the fuel rail. When stopping, the fuel pressure takes a while to drop back down to engine idle loading. As a result, the purge solenoid ramps up to return the excess to the tank.

Most cars will make some dull popping sound when this solenoid activiates (usually, from underhood). It is probably louder on the Crossfire since the tank is located so near the driver's ear. No cause for concern. By the way, you should feel good about saving the environment when you hear it!

Hope this wasn't too techie! Thanks. :roll: