webservice hosted on iis7.5 has one webmethod invoked every 5 minutes exactly by a "ghost"

I am a .Net developer encountering behavior of my ASMX webservice on an IIS 7.5 machine for the first time. It is a staging server so I know I am the only one using it.

Strange behavior as follows:

There are 5 webmethods in this WS meant to be called by web client code
(to my knowledge) there is no such thing as "default" webmethod for a WS
each one of the webmethods writes an entry to the Windows event application log upon entry
the event log entry shows which webmethod called and some other stuff
if I leave things idle, one of the webmethods gets called every 5 minutes EXACTLY

I've looked briefly at the rapid-fail protection feature thinking this might be it. It is currently enabled and set to 5 minutes but the other aspects of it don't seem to apply.

I changed it to DISABLED and restarted IIS but it still behaves in the above strange way. I am confused what this could be...

        [WebMethod(Description = "Search TRIM by parsing CF's WebDrawer string for search criteria")]
    public string SearchCF(string trimURL
        , string CFSearchString
        , string CallerPC = "not specified"
        , string RequestorID = "not specified")
    {
#if DEBUG
        string d = String.Format("SearchCF: trimURL={0}, CFSearchString={1}, Identity={2}, CallerPCname={3}, RequestorIdentity={4} "
                                            , trimURL, CFSearchString, GetUserInfo(), CallerPC, RequestorID);
        LogDebuggingInfo(d, 500);
#endif

Solution

This very strange condition described above was solved by installing the standalone version of .Net Framework 4.0 (64bit version) to this server. This was followed by installing ASP.NET 4.0 by running:

o   navigate to C:\Windows\Microsoft.NET\Framework64\v4.0.30319 
o   in cmd prompt (run as Administrator) execute:
  • aspnet_regiis.exe –i
  • iisreset 
o   reconfigure both the client site and the site hosting the webservice to run ASP.NET 4.0 (resulted in significant reduction of the number of entries in web.config

I checked out the other suggestions above before trying the above sequence which fixed it.