Our Vostro 1400 laptop is buzzing, but only when the monitor is DISPLAYING.  When it goes to black between programs or switching windows it does not make any noise at all.  Whats up?When this occurs, are you using the power adapter?  If yes, disconnect it and run on internal battery.  If hum stops, it's interference from the 60Hz power line.Nope, it still hums unplugged
 Screen refresh rate is 60Hz, but why it's audible is unclear to me.  Try changing refresh rate to 59Hz.
Found this, not sure how it applies to your situation.  http://davidvielmetter.com/tricks/howto-stop-lcd-from-making-humming-noise/
Thank you.
Tried the brightness, the refresh rate and downloaded the software another had tried and nothing made any difference Sorry to say, this MAY be an indication of bad things to come.  Take precautions & backup all important DATA onto external hard drive, DVD, flash drive, etc.  If the screen goes out, do you have an external monitor you can use?  If you do have one, give it a try & see if it buzzes, too.  It may be the LCD tubes are the culprit.  You've heard fluorscent lights buzzing?  Is it the same sort of noise?

I don't believe it's a driver or software issue, but here's a link to all of them:  http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=19&l=en&s=dhs&ServiceTag=&SystemID=VOS_N_1400&os=WLH&osl=en&catid=&impid=Components in video hardware (whether separate cards or onboard), capacitors and inductors mainly, can start to vibrate audibly as they age. If the sound changes when the screen content does it can be a sign of this. It can be a problem in TVs too. Sometimes a gentle prod or a spot of hot-melt glue can FIX this by altering the ACOUSTIC resonance, but this is really a repair shop job unless you know what you are doing.