Water in radiator resevoir is boiling?

I noticed while driving my car with the a/c on the water in the resevoir is boiling when I get to work. The engine is not overheating and there's no smoke, just boiling sounds. What causes this to happen? I thought maybe it was the thermostat but I'm not sure at this point. Also if this helps its a 1998 honda accord ex.

Kind of normal that a car purges the pressure into the reservoir then as the system cools it'll sucks back the coolant into its system/radiator. If you like get a new radiator cap. Maybe the old one is not hold enough pressure.