ao smith vertex GPHE 50 100

AO Smith Vertex GPHE 50 100, 2 1/2 yrs old gas hot water heater. New construction home. Unit is now knocking when making hot water. I drained tank, only a little discolored water came out. Knocking sound sounds like it originates in the tank but makes a knocking sound through the green plastic enclosure on top of the tank.