FleshAndBloodTheocracy
Living Apple
It sounds like a voice saying "Wa" really quick. Could be a scratch track or sliver of audio left on an open track during mixdown that should have been deleted. With all the vocals Matt has going on in all these songs it's probably very difficult to keep track of it all. What surprises me is the mastering dude missed it, especially since he's a big name and that's his job.
That's not mastering dude's job. Mastering dude's job is to equalize the tracks. That's pretty much it.
Production (choosing which tracks to use), engineering (assembling the tracks into songs) and mixing (choosing where the tracks go, volume/prominence speaking) were all done by Matt - when one person takes on all that responsiblity, it has the upside of coming out like he wants it to, but the downside of lacking someone else's perspective.
By the time mastering dude gets it, the tracks are likely all set in stone, more or less.