Just got a new road bike - rode it today for the first time, and loved it. However, when putting it away, I heard something rolling around inside the rear tire. What on earth could that be?

If it's rolling around inside the tire, it has to be in the tube, and I can't imagine how it got there. Could it be inside a hollow rim? Either way, now's a good time to take off the wheel, dismount the tire and look everything over. Better to learn today than by the side of the road when you have a flat.
One possible, but not likely, explanation: the hole for the valve will be drilled through both surfaces of a hollow rim, and it's possible for something to get in there. You'll play hell getting it out...

Will do. I'll let you know what it was after I open her up tomorrow.....

Is the little ring still on the valve stem? When they get loose they can make some interesting sounds. Should be ditched anyways.

Just went into the garage to take another look.

It's the piece that connects the spoke to the rim....

So the rim evidently has a hollow section into which this metal detritus has fallen.
You'll probably be driven crazy by the sound, and there's almost no way to get it out.

I'm taking it back to LBS.

I know ditching it is part of the rules but I never understood why.

I dont know the "technical" reason but if they are not tight, they make this kinda humming metal noise that drives me bat #$%& crazy

Ha wow I keep messing wwith mine and gettin all frustrated when they won't stAy tight, in chucking them!