You are right, xiphias. I have had air problem for the last 5 years - my house generally heats okay otherwise. It is just the noise and the feeling that there is still air (quite some) in the pipe make me uncomfortable.
As NJ Trooper explained, I now understand why sometimes I hear air sucked into the pipe through the bleeder upstairs while other times air actually coming out - the 'head' thing (pressure drop). I have all 3 zones on at times during the day and only the zone upstairs on during the night. The 3 zones might call for heat at different times (rare except for the upstairs zone) as they are independently controlled.
I think the air is moving along the water, not trapped. I think a inline air scoop (air purger as others call it) like this (
http://www.quality-stuff-for-you.com/info/AP400CAT.pdf) might help, but I need to know the best location to install this thing - on the returning line close to the circulator, or on the supply line going to the baseboards. And, do I need one for each zone?
Thanks.
Long