I can't tell what you have up around the expansion tank connection. I would suggest you install a Spirovent air eliminator. I had system I could not get the air out of, and I have a lot of experience with it. There was an old Amtrol air purge unit which I replaced with the Spriovent. 24 hrs later quiet as can be. If you try to do this manually it will drive you nuts.

Installing an air eliminator is an idea worth trying. I would say that in your case, the problem may be a bit more than what seems. Be cautious that any thing you try should make the situation better only.