I've learned alot about my water heater today and am posting what I've
discovered in case anyone else reading this thread is having the same
problems.

First, this HW heater is not available at Lowes or Home Depot, because
it is designed for MF homes and has a direct vent. Apparently this
makes it harder to find. Both of those stores recommended going to a
plumbing specialty store, which likely means a higher cost to replace.

But, I called the folks at Rheem, who are the makers of this HW
heater, and they said that the sound I'm hearing is probably related
to thermal expansion. They recommended installing a thermal expansion
tank. They also said that I should have a drain and flush done on the
HW heater. So, I guess if the sound keeps happening, I'll be calling
a plumber to do these things soon.

Thanks for the help,
Kim