hydronic heat - getting a big bang

In the house I purchased a couple years ago I have an old hydronic heating system with 3 zones.

Last winter and this winter I've started to hear a big "BANG" from the boiler. The noise carries through the pipes and can actually 'startle' you because it's so loud.

It didn't do it much when I first turned the boiler on this winter, but now it's doing it more and more.

Also, from a timing standpoint...it seems to make the noise after the boiler or pump or heat "shut's off". I know it's not happening when the thermostat is turning on...it's more after it's turning off.

I'm ready to call the local plumber, but I'd like a verification of what he may tell me.

Does anyone know what could be causing this big bang?

pictures of my system...

Also, when bleeding my system...should I use the bleedling lines attached to the baseboard radiators...or should I use the faucets attached to the boiler as you see in the pictures?

How do you know when to stop bleeding the lines?

Pictures of my system...
http://www.bruceschmidt.com/Photos/photo_frame.asp?Folder=\photos\Hydronic_Boiler