Solo110 with smart40 indirect hot water heater installed December 28, 2008. My smart40 indirect hot water heater is making rapping sounds once in awhile. The sounds go away when I shut the yellow shutoff valve at the top of the tank for the domestic cold water input. The sounds do not happen when any water hot water or cold water is turned on or off. Could this be an expansion problem or water feedback? Please note: The solo110 and the smart 40 are working properly and I have plenty of hot water within a few seconds. Also I can send a movie with audio so someone can listen to the sound. I found that the recirculating pump was set on low and all I did was set it on high and the sounds have gone away.