Buzz or Hum in Audio Port

Question

My MacBook Pro has a low-level hum or buzz coming out the audio port
whenever sound is coming out of it, and for 25 seconds more.  Then it stops.
The noise sounds a bit like the 60-Hz hum or 120-Hz buzz from tube amps,
but it occurs even when my MacBook is running on its batttery,
unplugged from its power supply.
The buzz is constant, independent of orientation of my computer, and sounds
about the same in all locations I've tried.
I can't hear it from the computer's speakers, but that's probably because
the built-in speakers' frequency response drops off above the most prominent
frequencies in this buzz.  I can hear it through Apple's earbuds  faintly. 
The buzz is most annoying through audiophile headphones or external speakers.
The buzz occurs with any application that makes sound.  Even system
bleeps will cause it.  After the sound stops, the buzz continues for 25 seconds.
Then the buzz stops.
I took my computer to the Genius Bar, but it was so noisy there that neither of us
could hear the buzz.  He took it in back where he used Apple earbuds, then reported
that the machine is "in spec".  Well, foo.
Any idea what happens inside the computer 25 seconds after the music stops?
I suppose I could try USB sound output.  Any recommendations?
--Gil

Answer 1

Hi, fane_j--
What MBP are you talking about?
Well, System Profiler puts it this way:
Model Name:
MacBook Pro
Model Identifier:
MacBookPro5,5
Processor Name:
Intel Core 2 Duo
Processor Speed:
2.53 GHz
Number Of Processors:
1
Total Number Of Cores:
2
L2 Cache:
3 MB
Memory:
4 GB
Bus Speed:
1.07 GHz
Boot ROM Version:
MBP55.00AC.B03
SMC Version (system):
1.47f2
It has just one 3.5mm + S/PDIF audio port, called "Combination Output" by System Profiler, used for either input or output, depending...
Intel High Definition Audio:
  Audio ID:    77
Headphone:
  Connection:    Combination Output
Speaker:
  Connection:    Internal
Line Input:
  Connection:    Combination Input
Internal Microphone:
  Connection:    Internal
S/PDIF Optical Digital Audio Output:
  Connection:    Combination Output
External Microphone / iPhone Headset:
  Connection:    Combination Output
Headphone:
  Connection:    Combination Output
Speaker:
  Connection:    Internal
Line Input:
  Connection:    Combination Input
Internal Microphone:
  Connection:    Internal
S/PDIF Optical Digital Audio Output:
  Connection:    Combination Output
External Microphone / iPhone Headset:
  Connection:    Combination Output
Does this answer your question?
--Gil