My wife's 2004 Altima (4 cyl) experienced the same symptoms (heater was not terribly warm, blew cold at idle, gurgling sound under dash (likely heater core) occassionally. The fix was SIMPLE. I parked the car on a steep hill, going up hill. I removed the radiator cap (let it cool first, or it will spew hot coolant & burn you - use a towel, too, just to be extra safe). Start the car and let it warm up. Rev it up to about 2500-3000 rpm and hold it there for about three minutes. Coolant will circulate and burble out of the top of the radiator - along with the air pockets that were in the system. Once this is done, top off the radiator with anti-freeze mix (use the right kind - (green) and use distilled water to mix it). Put the radiator cap back on and top off the coolant recovery reservoir. It took about ten minutes, about a quart of anti-freeze and no tools. It immediately began blowing HOT - even at idle. That's it! Very simple, very inexpensive. Obviously, your problem could be more involved, but this is definitely the thing to try first. It worked like a charm for me. The hill I used put the front wheels about 2-1/2 feet above the rear wheels (fairly steep hill). Good luck!