2007 Chevy Impala head gasket problems?

We have an 07 Impala with the 3. 5L V-6, 64,000 miles. Apparently the head gaskets are going out on these engines from 06-08; ours was losing a ton of antifreeze and the dealer replaced the head gaskets, crossover pipe gaskets, and the water pump under warranty at 61,000 miles.

I went to top off the cooling system yesterday and try and bleed it by running the engine with the cap off and heater on. Started the engine when it was stone cold and to my shock, there was tons of bubbling and fizzing like you'd get with a bad combustion to cooling system head/gasket leak. And when I rev the engine, the level rises instead of falls like what normally occurs. There are no other symptoms - no overheating, hasn't hardly lost any antifreeze since the repair, it runs great.

Have you guys ever seen symptoms like this that wasn't a head gasket problem? Could it just be the design of the system and placement of the waterpump compared to the filler neck for the cooling system (the cap is not located on the radiator), and I'm seeing churning coolant from the water pump?

Thanks guys for any info and help.