[edit Nov.16th 2009: Thank you for the responses. I'm no longer on this project so the problem is out of my hands. Be sure I will return with other problems :-) ]
I have a strange occurence of regular databursts over my WAN links from 2 sites to a site with vCenter.
What happens is that a vCenter server is managing local ESX 3.5 servers and 2 from 2 sites across a WAN link. Each server sends an approximately 3MB worth of TLS data (less than 10% of the time it varies to higher or lower) every 15 minutes (with a margin of 2 minutes). So far, I've not been able to single out a process that causes it. I looked through all applications on each site. So far, it seems to originate from one server on each site.
Although it may be coincidence and therefore not relevant, I found that one server, with very few exceptions, does a burst at 00:00 on the hour. The other 3 during the hour are a bit off the 15 minute mark but back at the top of the hour, you can sync your watch on it. The other server follows 5 minutes after that with no such precision. But, as said, it never differs more than 2 minutes.
Servers are ESX 3.5, vCenter is 2.5.