follow these steps precisely to the tee:
http://www.318ti.org/forum/showthread.php?t=27983 You have what's called air-lock and need to bleed your cooling system. If not done properly and in the immediate future, you run a high risk of overheating your engine, damaging sensors, cracking your radiator and other plastic parts as well as blowing your headgasket and/or warping or cracking the head.
DO NOT JUST ADD COOLANT AND THINK EVERYTHING IS OK. Yeah, you're probably very low on coolant, but you need to bleed it every time you top it off.