Chevrolet: cooling system overflow, hairline crack, 1986 z28 camaro

Question
Thank you for taking my question.                I have a 1986 z28 Camaro with a 305 ci tuned port engine and automatic transmission.
Just recently when I shut the car off I can hear the bubbling of the overflow tank being filled which is quite loud.
So far I have run the engine with the radiator cap off to see if it might be air bound. I pressure tested the radiator and checked the cap.
The engine seems to run at normal temperature and the thermostat seems to open at the correct time.
The overflow tank is at the correct level and I don't seem to be losing any coolant or have an internal loss into the oil.
This is a rebuild engine with about 6k on it if that could have any bearing on it.
Any suggestions would be greatly appreciated.

Answer
Hi Joe, I seen this problem a few years back, in this case it was cracked head, There was a hairline crack between the two valves on one cylinder and it was forcing compression out into the coolant. It was undetectable to the eye when the engine was cold. We had to take the heads to a valve shop whrer the had special dye to find it, anyway we replaced the head and it was fine. I hope this helps you if you need more info just let me know. Regards, Keith.