The LiveUpdate running post-install is by design so that the time which the clients go with out of date definitions is at a minimum. Unfortunately the default is not silent--likely also by design so that the user knows that it is running.
If the Mac clients are managed, then their LiveUpdate schedules should be silent by default. If they are unmanaged--they should have a default, built-in schedule for all users that is also silent (there was an issue on some SEP11 builds with the default schedule not being set to silent, but that has been resolved.) You could check on an end-user's mac (sudo symsched -l (lowercase L) in a Terminal window) to see how it's scheduled. The link pete_4u has provided above has more details on the commands.
sandra