Bubbling air in the reservoir

My wife drives a 2001 Chrysler Sebring Limited convertible with 105,000 miles on a 2.7 Liter, 24 valve DOHC. Afrer driving for any extended period of time, I have recently notice that there is a bubbling sound coming from under the hood. When I raise the hood, I notice the sound is coming from the Radiator water reservoir. At first I thought the water was boiling but after a closer look, it seems like mabye short bursts of air are going into it and making the water bubble. Each bubbling sequence lasts about 2 seconds or so and they are a couple of seconds apart. The car does not seem to be overheating as the dashboard temperature gauge stays just below the 50% mark throughout the drive. We are running the air conditioning while driving. Any ideas what could be causing this?

Thanks for your help,
Dennis
Memphis, TN