Joey is absolutey right, however in my case it was a little more complicated than that. You probably have a small leak some where in the cooling system, so when the system is cooling off after you dirve it is sucking in air through the leak. This air alway s seems to get traped in the heater core under the dash. I was able to get rid of it by unhooking the heater hoses (the one that run from the enigine to the firewall) and then hooking a garden hose to the lower heater hose by using a hose barb that fit onto the garden hose. (these are available at home depot as a replacement end) Then turn the water on slowly and flush the air out of the core. then reconnect the heater hoses as quickly as possible to keep the core full. I haven't had a problem since, ( I never did find the leak, hopefully it's not into the cylinder). It just seems like for some reason the air traps in the heater core and it is a pain to get out. You could have the system flushed by a shop but I was afraid a pressure flush would make my leak worse.