This ins't a problem as such, more of an observation. I have three active DNS servers on my network; two live and one which is soon to be decommissioned.
While checking the DNS logs of the soon-to-be-decommissioned server, I noticed the PRTG server is querying "localhost" to this server every 60 seconds. This is odd because this DNS servers IP address is not listed as a static entry on the PRTG server, so I'm not sure how the PRTG server is finding it. It is currently still authorized in AD, so maybe it's finding it from there.
Either way, it raises the question of why is it querying it at all? and why so often?