I've only really experienced this with GM products and I assume it's partially to do with the fact that most GM coolant systems operate at 20+ PSI (higher than most other common manufacturers).
The pressure alleviates overnight (or when the car is off for a long period). My shower theory has been that the relatively high PSI, combined with thermal expansion of the coolant, then the simultaneous (gradual) loss of pressure and thermal contraction of the coolant can allow air bubbles to form in the heater core/elsewhere if parked at certain angles.
I've tried bleeding past GM systems of air but it never helps. I've heard it in my TB on occasion (particularly this past month with the cold weather) , as well as my Cruze before, but never as prevailant as with the Pontiac grand am I had years and years ago. In short, wouldn't worry about it, just a little weird quirk with GM engines. As long as it's not a persistent noise then the air is finding it's way out and not being trapped in the loop.