On the hot water heating pipe above my boiler, there is a green elbow-like joint with a small valve on top. I've been having clanking and swishing water sounds in my baseboard, so I'm guessing there's a bunch of air in my baseboard lines.

I've tried to bleed the lines from the baseboard valves, but I never get any air coming out, just trickling water. My uncle says it may have to do with the valve over the boiler. He says you are supposed to leave it a little loose to allow air to escape. When I went down there, it was as tight as it could go. It is safe to loosen this a bit and leave it like that?

Also, could the reason the air wasn't bleeding out was that this valve was tight? Does it have to be loose in order to bleed the upstairs lines?