The gurgling sound coming out while your espresso maker is on the stove is due to its steam pressure brewing system. The water in the lower chamber gets heated until it becomes steam and is forced above the filter basket and into the bed of coffee grounds. As the steam extracts coffee, it goes up the spout and then finally drips into the top compartment of the Moka pot, where it gets collected. The gurgle is just the sound of steam moving through these three chambers on its way to your cup.