Glad I'm not the only one who hears this, then. I didn't think it was guitar-tapping or anything like that, since it sounds too distinct, but maybe. Either way, not really worth complaining about, and nothing we can do about it. (Not the first time a Rick Rubin-produced album had some sound issues, either: http://en.wikipedia.org/wiki/Death_Magnetic#Criticism_regarding_production)

When I listened to the NPR preview of the album, I noticed that it had some weird popping and skipping noises which made me think it had been ripped from the vinyl. I wonder if these clicking noises showed up on that as well, if it did come from a non-digital source. Guess there's no way to find out at this point, though.