OK, I am back from the dealer with my car, noise free...but it isn't that simple.
One thing I neglected to mention before, is that the thing which really prompted me to take my car into Carmax was the Service Engine indicator came on. The rattling sound had been going on for some time - I had almost gotten used to it - but the Service Engine indicator was what I brought the car in for.
When I brought the car to Carmax, they took care of the Service Engine issue (I'll get to that in a sec) and had them check the noise. That was when the timing chain diagnosis came down.
Coincidentally, once I picked the car up from Carmax there was no clicking/rattling. NOT A SOUND. For one full week there was NO RATTLING AT ALL. Hmmm...
Today I dropped the car off at the Nissan dealership. They did everything they could to hear the noise. The service manager said they "put a stethoscope up to it" (I am guessing that's hyperbole) and couldn't hear anything which would indicate the chain was going bad. She DID say that it is very possible that when Carmax took care of the Service Engine code, which was for an IDLE AIR RELEARN, that took care of the problem.
Well, that's all I've got! I am sorry that there isn't a specific issue I can laser in on, but at this point I am just going to go by what the dealership says, and the incredible coincidence of the sound disappearing immediately after the idle air relearn service.