[mrcaptainbob]

Ah...HA! Although this goes against everything I am, I broke down and looked at the book that came with this heater. In the "Before You Call For Service" section:
Rattling noise during periods of water usage ~ Internal heat traps in operation ~ This is normal for heat trap fittings when in operation and does not indicate a need for service.
So there it is. Hidden in the book of directions. No wonder I never saw it!