White Smoke After Idling

Hello,
I have a 2003 Subaru Impreza WRX with a turbo back exhaust and an up-pipe. It only has about 40K miles on it. I recently noticed that after I let the car idle for at least five minutes, a cloud of white smoke will appear from the exhaust when I rev it. After reading some of the forums, I am concerned that it is the head gasket'; return true;" onmouseout="window.status=''; return true;">head gasket, but it doesn't do it on start up or pretty much at any other time, only after idling for a set amount of time. Sometimes it won't even do it at all. The cloud will onlly last for about five seconds and then it is gone. What do you think that it could be? Any advice is helpful. Thanks. :D