I'm curious what's causing it. My guess is either the NFC parts are sourced from more than one company, and one is faulty, causing the sound. Or there are just defective parts among the parts from one supplier. I don't think a software issue could fix it. We're all on exactly the same software, some have it, some don't, so I suspect it's a physical defect. The issue could even be with the speaker, since some people have that high-pitched whine too. Maybe the speaker is making audible the NFC ping. Is there even such a thing? Google support had me plug in headphones to see if the sound was transmitted to them. It was not, and still audible from the phone even with headphones plugged in, so it's not a sound being purposefully played over the speaker.
By the way, called in to Google support. They told me that the RMA for this issue is considered a new phone DOA, and therefor you have another 14 days from receipt of the replacement device added to your return window. They said they can't confirm this until you've received the replacement unit, but if it's true, at least you have another 2 weeks to make sure the replacement doesn't have issues as well.
He also said that Google is starting to consider the NFC clicking "within acceptable standards" of the phone, so RMA's may start becoming hard to come by if you're on the fence about getting one.