Popping and Banging in a Combi-Core

A customer has a 73 gal, 76k btu Combi-Core heater used for domestic hot water with a radiant system thru the internal coil. It's about five- six  years old, on a well with probably hard water, and it began popping during the heating cycle a few months ago and is getting louder with time. I have heard this same noise before, my own two 40's used to do it before I put in an indirect. A sudden draw (like the automatic washer) will trigger it more. I presume it's scale on the water side of the flueway? Any expert advice on a cure (if there is one) would be welcome!