HTTP request in Server log for the website page in every one second

Hi,
I have a website running on the IIS 6.0.
Recently, I've noticed that there is a record in the server log which make the request to the default homepage of the website itself every one second. And the IP is the IP of the current website.
It is not a CMS, we don't have "keep alive " things or some other else.
It happened only a few months ago. And I've no idea what setting has been changed or modified in IIS or something.

Any idea appreciated.

Thanks.