I've noticed that a couple of apps seem to like to use the sync technology -- or at least they display the sync symbol occasionally, whatever technology they are using. For the life of me I can't remember any right now. But, if you install "Watchdog" from the Market, a process monitor, you'll be able to see all the processes and probably be able to determine which app is "syncing". After that it will probably be a simple matter of going into the app settings and turning background updates/syncs on or off, depending on your wishes.
Watchdog is only one app that makes it easy to see the processes, there are many. Also, you might even find it in Menu | Settings | Applications | Running Services | Running.
-Frank