boiler problems

We have a four year old Unical CTFS 28 boiler. This week it started making noises like a very active popcorn popper when the heat was on. We couldn't figure it out, so got a local plumber who works on these things to come to the house. He thought it had something to do with calcium buildup, or with breakage in the circulatory system during the winter. He recommends replacement with a Saunier Duvall system with an output of 18,000 calories (not a measure I know) at a total installed cost of E2000, Does any of this make sense?