Web.config Always Reverts to authentication mode="Windows"

[DaBrain]

I am new to ASP.NET (Was doing ASP) I am even newer to Visual Studio.
Every time I "Publish Web Site" (doing so on a local networked
drive) after the publish is complete I get "Web.config was modified
outside of the source editor."
So it is modifying my local copy of web.config before it publishes
the site to the networked drive then informs me it has done so? I
assume it is Correcting an error on my part.

It changes
authentication mode="None"
to
authentication mode="Windows"
What am I doing wrong here?

Many thank for any replies in advance.