What is That Noise?

At first, I thought it was a sign of old age. I heard it during the Fenway Park games of Boston's ALCS series with the Cleveland Indians.

Then, during last night's World Series opener pitting the Red Sox and the Colorado Rockies, my son turned to me and asked, "What's that sound?"

I'm watching the game tonight and the noise is there again.

What is it? It sounds like chimes, a low-volume, background of incessant ringing.

Joe Buck and the ever-quick-to-educate-us Tim McCarver haven't mentioned it. I've Googled everything from "Fenway, Boston, ringing" to "Fenway, Boston, chimes" and "Fenway, Boston, noise." Nothing.

Does anybody know what that is?

[UPDATE: For the record, I love the percussing relief pitchers in the Boston bullpen!]