If you can press on the back it reduces the buzz tightening the screws may work. The best way to tighten them is to loosen the screws while the tv is off (cold), turn the tv on and let it warm up, and then tighten the screws back down while its still warm.
If the buzzing is loud then there's a problem with a component inside (can't recall what exactly) and the tech would have to remove the back panel to check it. Darned buzzing Samsungs probably have all these techs thinking it's normal to be loud. It isn't. If you do a search on the Panasonic buzz and look for Randy's response you'll find more detailed information. It may even be on this thread. If not then the ST50.
Pushing on various parts of the back does nothing. One thing I hadn't noticed is how staggering the buzzing difference is if I'm a few inches away from the TV and the screen switches from a mostly white scene to a dark scene. On a dark scene I can't hear any buzz if I stick my ear right next to the back (I hear the fans a bit at this point).