The wisdom of Click and Clack, the Tappet Brothers, is applicable here:


Turn up the radio.


Squirt glue into the rim, and depending on how far the noisy bit is from the valve hole, there's a good chance you'll lock the nipples in without catching the shard. You won't be able to re-true the wheel, and it'll keep rattling.


I'll vote for the whack, shake, and repeat until it works its way down to the valve hole. You'll want to think about how you're going to grab it to get that sucker out. When you turn the wheel upside down, Murphy's Law says it'll miss the hole and head 180 degrees around the rim. Small needle-nose pliers, tiny tweezers, or maybe warm, sticky bubble gum on the end of a toothpick might work.