Air in coolant system

I own a 2000 gmc sierra 1500 and have noticed this noise in the cab of the truck now for quite some time. I beleive it is air in the coolant system because it happens when the vehicle warms up and sounds like it is coming from the heater core area, passenger side dashboard. I have tried replacing the radiator cap with another brand (stant) but the problem still is present. Does anybody have any other suggestions.

There should be a bleeder screw somewhere on the block. You need to crack it open to let the air out. I am not sure where it is but there is one on the engine.

//2000CAYukon