I have an oil furnace that also has a small resevior of for the hot water.
When ever it is running I hear a boiling sound which can be annoying because
you can here it in some rooms rattling away in the basement.

I don't believe it is air in the system because the rattle is coming only
from the furnace and not the loops on the first or second floor. The water
seems to flow without a problem through the system and the pipes in the
rooms don't bang or anything other than allow the sound from the basement to
travel through them. Also, when I open one of the air purgers on a pipe
that rises straigh up about 4 feet above the furnace there is no air in it.

I have drained the water out of the system entirely recently because I had
to change the circulators but almost immediately the problem returned.

I do see green deposits building up in a lot of spaces like down the pipes
from the air purgers down so I was wondering if there was some buildup in
the tank and if their is a way to get rid of it or control it.