Forced hot water....lil' help?

I've got forced hot water in my home. The lines have been in need of an air bleed, and the clowns that came to service my heater didn't bleed them, so I tried doing it myself. There's a couple valves that look like they're near what appears to be the return pipes to the boiler. I opened them and yeah, got a bunch of steam, sometimes just air, sometimes just water, sometimes a combo. And since I tried that I've been getting this annoying rattling sound in this green 90� elbow that says "TAOO Universal Flochek", which is located downstream of the valves right before going back into the boiler. So my questions are: Am I bleeding the lines properly? What is that green elbow, and is there anything I can do to get rid of the rattling sound?