boiling sound in furnace

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.
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Furnaces don't heat water, they heat air. Boilers heat water.
"Motorola Phone" <news.lightship.net> wrote in message

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First thought is the circ isn't running.

You sure you have the right circulator? Does it heat 10 feet from the boiler? Is the new Circ running or is there a valve turned off?

The green stuff is probably the leftover flux from the soldering.
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<<Furnaces ...heat air. Boilers heat water.>> OK
<<First thought is the circ isn't running.>> All zones seem to heat well. Hot water seems to get to the end of the longest loop very quickly and the small zone on the second floor gets heat without a problem. I don't hear any gurgling in the pipes when the just the circs are running.
<< You sure you have the right circulator? >> To be honest I don't know for sure, but my type of system is common in New England and most everyone has the same size and HP circ as I have on each of my zones.
<<Does it heat 10 feet from the boiler? >> No issues with getting heat where needed.
<<Is the new Circ running or is there a valve turned off?>> All throw valves attached to the unit are in the open position so water is flowing freely.
The problem is not with the ability of the unit to heat the home, it is just that when the the water inside is being heated it sound like it is boiling, but not exactly like a boiling pot. I guess a better description would be it sounds like a sealed coffee can filled to the top with water and with a few wood beads in the can being shaken. You don't hear water splattering sound or whooshing but you can hear a non-metalic tapping going on.
This unit as done this since I purchased the house. Recently I had to do some work on one of the radiators so I thought when I totally drained the system it would flush out something, but it still happens even after the flush and fill.

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