You could try leaving dbus-monitor running in background and check the output when you notice the vibrate:
1. Start X-Term from the app menu (or wherever)
2. Type "dbus-monitor<enter>"
3. Minimize it and do whatever you like until you notice the strange vibration
4. From dashboard open the X-Term
5. Ctrl+c to stop monitoring
6. Scroll up to look at the entres above and try to see if some of them look like your vibration thingy. Could be something about "notification" for example.

BTW, You can get more readable output from dbus-monitor by adding line feeds before every line not starting by space:
dbus_monitor|sed "s/^\([^ ]\)/\n\1/"