Intermittent brake grinding, Mazda3

I drove 900 miles in the mountains this weekend for vacation and then, just a few miles from home, I started hearing low frequency grinding sound when I stopped at a light. The next four times I stopped, no grinding. Then, back again for the next stoplight, then nothing. I plan on taking it in to the dealer, but what if they can’t reproduce the grinding? Any ideas?

It’s an '05 Mazda3, 30K.

Not at every stop though. More like a quarter of all the times I stopped, I heard a low

Possible worn wheel bearing.