I get a message like this in my apache error log every few minutes:
Jun 29 11:13:37 www apache2[22839]: [error] [client 129.130.453.734] ping, referer: https://www.example.org/user/262725/downloadhistory
The referer is always a PHP (Drupal) URL, but that may only be because the site is almost all Drupal anyway. I can't find any documentation for "ping" that doesn't refer to a network ping.
This sounds like it's probably related to the Drupal Ping module. See here.