When switching tires/wheels (summer/winter), I noticed a noise inside my tire that sounds like small marbles rolling around.

What could that be?

Some fancy 'self balancing' gizzmo?
Foreign particles (not sure how they might have gotten in)?

Any suggestions, explanations?

 

could be parts of old valve stems. It is common practice to break off the old ones when changing tires on rims, maybe they did not remove the little round rubber piece that breaks off

 

I asked discount tire: they said this happens if extra 'grease' (lube put on when mounting the tires) comes off. Rotation forms the grease into ***** that harden.

They see that ones in a while. Nothing to worry about, and really nothing to avoid it. Just happens once in a while.