right after cool down for 3 minutes controlled by the greddy turbo timer......

The key was stayed on ACC. There was no light and beeps while during count-dowm. Once the engine is off, the light and beep were on. The stock coolant temp guage was pointing at 9o'clock. I had experienced this once before. I was scared, so I turned the key to OFF and then turned the key back to ACC and started my car. The light and beep didn't go on anymore. So what's the matter with it??
By the way, I can barely hear boiling coolant when the hood is close. I suspect there's air trapped in the coolant system.

Plz help.