pkguy, once your system is bled and totally filled with water there should never be a reason to have to bleed in again. Are you sure it air that you hear? One indication would be a gurgling sounds in the baseboard elements when the circulator is running plus a definate temperature difference when feeling the inlet and the outlet piping at each section of element.

When we first moved in last November there was just an ever slight sound of gurgling upstairs in the bedrooms, a few seconds and gone. No water sounds anywhere else in the house, it's a four level side split level. My pc is down here in my workshop right next to the boiler and whenever the furnace ciculator pump started up I never heard any water or gurgling sounds, now in the last week or so there's a definite rush of water happening each time and you can hear it on the living/dining room level as well.

You can just see the air expansion tank, reddish color, up top. It has a hollow ring to it when I banged on it.
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A good place to install an air vent is right into one of the tappings on the top of your boiler. You can T off the pressure relief valve and intall one there too, with the relief valve at a 90 degree angle and the air vent upright.

One thing I'm wondering since the air bleeder valve will be down in the basement above the boiler. If after the plumber installs it and turns the water and pump back on will that circulating force all/any air that is way upstairs in the bedroom radiators to keep moving thru the loop till it gets caught and expelled by the new air bleeder valve. Or will he still have to go upstairs and "let some of the air out" upstairs? I'm trying to imagine how large air pockets or bubbles are going to make it "down" some vertical pipework and back to the boiler. Make sense?

The person who does the work should know how to properly purge your system by shutting off certain valves and raising the water presure manually then restoring everything back to the way it should be. I don't recommend a layman do this. One little mistake and you can get some kind of serious burn from 180 degree water!

And that's exactly why I'm not doing it myself. I'm going to call a heating/plumbing co. that's been in business here for decades still under the same ownership and insist they send someone who knows this stuff forwards, backwards and inside/out. I'm sure any of them can come in, cut and burnish and slide in a valve and solder it but it's the purging and filling, sequence etc I want done correctly so I don't end up calling them back or having problems because he didn't do it right the first time..

A system with a expansion tank like yours should not have an automatic air bleeder. The system should be piped so air is removed from the water and stored in the tank. It looks like your system is piped correctly. I recommend that you have your water feed checked.

The ceiling hung expansion tank will remove air as long as the pipes are pitched correctly. That depends upon the quality of the installation and many aren't. An air vent at the top of the boiler will serve as a second option.

Am I getting conflicting messages here? Why could I not install an air bleeder valve on an expansion tank system? Or why could I?

No conflicting messages. Adding an air vent in the top of the boiler could be the cheapest solution to your problem.

Here is a picture of the source water line for the boiler feed up there by the rafters. The tap is fully open. What is that first valve to the left of tap? Note is has a little lever on top pointing horizontally to the right. To the left of that is another valve with a different type of lever, this valve also has an outlet running down to the floor so I'm guessing it's an auto overflow valve of some sort. Can anyone tell me by looking if that first valve is open or closed by that little lever? I'm hesitant to even touch it for fear that water will start spraying everywhere.
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This picture shows a combination pressure reducing valve and relief valve. With the stem on the reducing valve on the right in the downward position, it is factory set for 12#, although it can be adjusted. Moved to the upright position, you'd have full house pressure to bleed lines. Always keep the valve at the reduced pressure to prevent the relief valve from blowing off.

The relief valve on the left is the inline type and the pipe coming out of the bottom probably goes to the floor or a drain.

They are sold as a set.

Thanks. I had a heating contractor in yesterday to install the air bleed valve and he also checked the boiler over and explained it all to me which was quite helpful. The pressure is back up to normal and all is quiet now when it's running plus I have good heat radiating from all the baseboards. Here's a pic of the air bleed valve. He mounted it atop the vertical leading out of the boiler. On startup you could hear all the air escaping.
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Glad it's fixed now. Your piping to the ceiling hung expansion tank must not be pitched correctly, but you'll be OK now. Sorry I didn't answer sooner; it kept slipping my mind.