Why do fluorescent bulbs hum/buzz?
Short answer: this is part of the nature of fluorescent lights. This is normal behavior so you probably don't need to replace the bulb or replace anything in the fixture. 

More can be found from the freely available information on this well-understood phenomena: 


  What causes the buzz? Basically a fluorescent lamp is a glass tube
  filled with an inert gas and some mercury. Filaments in the ends of
  the tube are turned on briefly by the starter to vaporize the mercury.
  Meanwhile, a voltage is applied across the tube to ionize the mercury
  vapor, allowing a current to flow. The ionized vapor emits ultraviolet
  light in response to the electrical current, which in turn causes the
  material lining the inside of the tube, called a phosphor (actually a
  mixture of several phosphors), to fluoresce, i.e., emit visible light.
  Don't worry, we're getting to the buzz. Once the gas in the tube is
  ionized, the current through it rises so rapidly that, if it were
  allowed to continue unchecked, it would reach dangerous levels and
  trip the circuit breaker. To avoid that, a device known as a ballast
  is used to limit the current through the tube. The ballast is your
  culprit here.
  
  Prior to the introduction of quieter, more efficient electronic units,
  a ballast was an electromagnetic device called an inductor. An
  inductor consists of many turns of wire wound around an iron core and
  operates, as one might suppose, by the principle of induction. A
  change in the flow of alternating current through the inductor creates
  a changing magnetic field (ever built an electromagnet by winding a
  lot of wire around a bolt or nail? Same idea), which in turn creates,
  or induces, a changing voltage in the wire, retarding the current
  flow. The upshot is that the inductor acts as a sort of electrical
  brake, limiting the amount of juice passing through the tube.
  
  Under certain conditions, the magnetic field can cause an effect
  called magnetostriction, which means that the magnetic field
  physically squeezes the core, altering its shape slightly. Since the
  fluorescent fixture is operating on alternating current at 60 Hz (that
  is, 60 Hertz, or cycles per second in the U.S.; 50 Hz in many other
  countries), the core gets squeezed and released at twice that rate, or
  120 Hz, resulting in the buzz you often hear. That's it, really. No
  eavesdropping, no attempts at extraterrestrial communication, nothing
  to worry about. Just don't start talking back.


-The Straight Dope

For replacing an old electromagnetic ballast with a new electronic ballast, see below. Although if you have a stand-alone lamp you might as well just replace the lamp fixture itself.

How to Fix Fluorescent Light Humming -wikiHow