I've got a 3-zone baseboard/fin-tube hot water system in my house and
hear the water flowing when it circulates. From reading some other
internet-advice, it appears that I need to bleed the air from the
system.

I've found the valves for the loops and understand that I need to bleed
from the highest point. However, I still have a couple of questions...

1. Some reading indicates that I should bleed when the water is
circulating, other places mention that you should not do it at this
time...the theory being the air settles at the highest point and
therefore you can release more when the water is not circulating.
Which is the better option, or is it a personal preference?

2. If the air is removed, how does the water enter the system. Is the
bell-looking device a control valve? I can reach this object, but it
is in a location where I cannot read the text on it. Is there another
valve that controls input flow and/or some type of auto-fill mechanism
that would allow water to come in when the necessary?

Thank you,
Dave