I guess the only way to find out who is contacting the DNS servers is to sniff it.
One thing I'm wondering... at the end of the page, it says
"If you are using a rpoxy server, then be careful about using 127.0.0.1, which may overload the proxy."
Why would it overload the proxy?
If a source queries a dns server which points them to 127.0.0.1, it will simply try to connect to itself.
Unless... the proxy is handling the web requests first and then the proxy will query the dns servers and resolve to localhost on itself. In this case using 0.0.0.0 would be better than using 127.0.0.1.