Hi,
Can I ask if the buzzing starts right away or after a certain time once it has warmed up.
In older CRT tvs most of the heat was internal to the workings and sometimes would buzz once the temperature inside (usually the transformer) got to a certain point.
In modern flat screen tvs the main heat source is the front of the screen, so it may be that one or more of the fixing screws holding the front and back together may be loose. Once your tv is buzzing try to isolate the source by gently pressing on the back panel to see if it stops. If not then there is something inside which may be loose, do not open up the tv, leave this to a professional.
You say the tv is on top of a unit or cabinet, this can act as a sounding board amplifying a small buzz into somthing louder. If the tv is not too heavy try to lift it an inch off the unit and see if that reduces the buzz. If this is a solution then place a sound insulating pad (such as a mouse mat or folded tea towel ) under the tv. I've done this myself when my disc recorder started to vibrate two cheap mouse mats did the trick. The recoder still buzzes but at a much lower volume than before.