Gurgling sounds from the center console when accelerating

Hi,

Recently, I've been hearing bubbling and gurgling sounds coming from the dashboard (center console) of my AJM. Specifically, it happens when starting the engine and during the initial acceleration, as well as during acceleration in general.

I then tried to bleed the cooling system (by running the engine with the heater on and the coolant reservoir cap open).

While doing this, I noticed that every time I pressed the accelerator, bubbles would come out of the expansion tank (air bubbles).

Could this be an indication of a faulty head gasket? (Air is being forced from the combustion chamber into the cooling system), because somehow the air has to be getting into the cooling system.

Hello,

This strongly suggests a faulty cylinder head gasket.
How many kilometers are on the engine?

121,000 kilometers, that's a bit early, isn't it? The engine isn't tuned, and I'm more of a fuel-efficient driver who compares diesel prices. The engine doesn't really get up to full speed.

Does anyone know how much the repair will cost? Is there any possibility of getting a discount (he's 3.5 years old now)?