I have a new home and need to bleed the boiler system as it is too noisy. I have checked the side of the vent and it seems to have some kind of small special tool that I have never seen before that fits like an backward allen wrench. How do I work it? Do I just turn it to the left to let the air out? Do I do that to each vent? Or do I have to shut the system down? I am afraid of breaking it. I think it might be a keyed vent (?? )

And where exactly do I add water to the system again? In the basement where the boiler is located? Help! I need details. Thanks!

Yes you add water in the basement, but it should have a self fill valve. Odds are you need a bleeder key for that bleeder. Look on all your vents there may be one somewhere. If not you should be able to get one at any hvac store. And you can bleed it with the unit still on.

If you look closely at the bleeder stem, it is most likely square. You should purchase the key, but you can also use a pair of needle nose pliers to open it.

Yes turn it to the left to open and let air out. But make sure to have a small cup or pan handy to catch water that will come out. Sometimes you have to bleed these for awhile before you get spurts of air. Bleed it until you don't get any more air with water. And yes, I would suggest bleeding all registers.

As Josh stated, your boiler should have a self filling valve, so you shouldn't have to worry about adding water. There should be a pressure gauge on or near the boiler. Check this gauge reading before you start to bleed system, and then check it again when your done. It should read about the same, within a pound or two. Please let us know how things work out.