I have a 12 year old 6 section cast iron hot water boiler. It has started making a perculating noise anytime the main flame lights off for the 1st 30 seconds or so. I suspected a scale build up but when I blew down the boiler it came out crystal clear. I put in the correct amount of 8 way boiler treatment and it made no difference. The pressure is good at 12 lbs water pressure. Any suggestions on what I can do to eliminate this noise? I am afraiad the boiler is goingto crack.