That's the vent hole so air can come out as water displaces it inside the tank. That gurgling noise you heard is common, the tank is getting full, or there is a bubble in there that is restricting the water. Without running back & forth between the hose & the tank sensor, the only way to tell when it's full is when water starts coming out, usually from the same hole you have your hose in. It's like when you fill your vehicle with fuel, when the tank starts getting full, you can hear a gurgling noise. Sometimes the there is too much line and an air bubble forms and the tank will act like it's full, when it actually isn't. Mine has done this a few times & it was only 1/3 full. I have a short section of tubing that I attach to the water hose & put it inside the fill tube. If water starts coming out & I know it's not full, then I've got that bubble. I'll turn off the hose (I've got a valve on the end of the hose) take the tube off, & put it up against the vent hole & blow through it to clear the vent. One time will usually do the trick, then reattach the tube & continue filling the tank. I probably should tear into it & see if I've got too much hose back there, or need to reroute it to prevent the bubble, but I haven't been able to find that "tuit" (round tuit)! It hasn't been that much of an issue because most of the time we camp with full hook-ups.