A couple of things I can think of (I'm not a pro). It can be a pain depending on the system design to get all of the air out. Does your system have an air purge valve? One brand I'm familiar with is called SpriroVent. This goes inline and sort of 'combs' air bubbles out of the water stream and releases them out of the closed system. You may have something like this and/or one of themore common types. You system should also have an expansion tank. Make sure that it is not full of water. Tap on it, the top should sound hollow, while the bottom, having some water in it, won't. Sometimes, these things are inverted - the side away from the inlet should have air in it. If it is full, it needs to be replaced.
If you have a good air extraction system, eventually, it should purge all of the air out of the system, and it should quiet down. In the interim, if there are any purge valves at the highest point, try opening them and see if you get any air out of the system. There's probably a purge valve at the boiler, too, try that and see if you get any air out.
Air in a closed system is not a great thing. the sooner it is purged, the better. Once all of the air is out, opening the valve should be nearly silent.
My unprofessional opinion...