[Zope] Z2 Log question

[Stuart Foster]

I was looking at my Z2 log and notice an entry that repeats itself over 
and over and was wondering if anyone could tell me how to trace it down.

The message is

 [20/Nov/2000:13:00:11 -0700] "HEAD /opnix/portal HTTP/1.0" 401 1906 "" 
"" 
 [20/Nov/2000:13:00:17 -0700] "HEAD /opnix/portal HTTP/1.0" 401 1906 "" 
"" 

and as you can see it happens about every 5-6 seconds and there are many 
entries for it. 

Where would I start to back track this. 
We are running Zope behind Apache using proxy pass.

Any thoughts.

TIA Stuart