Heater Blowing Cold Air at Idle

Coolant level is low and is not circulating well. You're hearing the air flowing through the heater core. The water pump has to spin faster to build enough force to push the coolant through the heater core.

If the level is okay, where did the air come from? The most common suspect would be a leaking cylinder head gasket. To verify that, your mechanic can do a chemical test, but GM made that somewhat difficult by not putting a cap on the radiator. Air from the radiator is drawn through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

You can often borrow the tool from an auto parts store, but since the fluid is rendered useless if it gets contaminated with coolant, they make you buy your own bottle of fluid. It may be less expensive to just have a mechanic do the test.