Problems with HP-L2335 23" Monitor

In October last year I've bought a 23" TFT monitor and was very happy about it. 1920 by 1200 pixels forming a beautiful picture... Then the summing, buzzing noise started. After about 10min runtime it sounds like it looses some internal sync or something. The picture is still fine, but the kind of vibrating, nasty, humming sound paired with some higher frequencies difficult to recognize, give me a headache. I've tried power connections in different rooms at 3 different locations and fed the signal with the notebook (even though the display menu without any signal is enough). No change. So I've called for a replacement...

• replacement 1
  1a support-response, the next day I've received a new device... but except the power-on LED no picture at all...
• replacement 2
  Again a fast delivery. The humming starts right from the beginning and seems to be even louder.
• replacement 3
  Again the same humming sound. In all the deliveries a German power cord was included, so they figured maybe I use a unsuitable Swiss power connection.
  
• replacement power cord 1
  Again a German connector.
• replacement power cord 2
  Unexpected and without notice. But hey, it fits my tablet PC ;)
• internal (Customer Relations Team, HP Switzerland) delivery 1
  Yes, it makes a strange noise, but we can't tell with only one test device...
• internal delivery 2
  It supposed to be alright, with just some "normal" noise. Forwarded to me as the next replacement.
• replacement 4
  The sound again... Maybe this is normal!?!
• more detailed research on the net
  And really there are report about other monitors emitting a humming sound. In most cases the replacement has been fine. Some even figured out some good and bad production series (unfortunately from another brand).
  If you increase the brightness to 100% the sound vanishes.
• response to my research
  Serveral internal personnel confirmed, in a "normal" environment no sound can be heard. => No further support is offered. No changes to the product.

Thank you very much for redefining my "normal" environment! After confirming the problem in the first run!?!
Well, it looks like I'm stuck with a pretty bright working place at day and night (I rather have full light on, than having to wear headphones all the time).
I hope some day there will be a test or regulation for monitor noise levels or something alike, so I can choose my next viewport to my data more successful.