AO Smith Vertex GPHE 50 100

(Skillman, NJ)

Natural Gas Water Heater 921187500 installed in Model Home 2009? Date on Heater is 05/08/2008 but Recent purchase of home. Jan 2015
Makes KNOCKING noise when heating water. (In addition to exhaust fan which is normal for High Efficiency water heater. The Knocking is coming from inside the water heater (not the fan or exterior pipes or PVC exhaust) This does NOT seem normal and does not appear to be a problem heating water? Is this water boiling in bottom of tank and overcoming pressure?