It appears that C427 and C418 are 2.5V electrolytic capacitors. Their low voltage rating suggests that they are associated with a switch-mode regulator that supplies power to the CPU. Notice that the inductors, L30 and L31, are very close to the place you identified as the source of sound. I suggest that it is far more likely that the noise is coming from one or both of these inductors. This because the ferrite that forms the core of these inductors is a material that exhibits magnetostriction. That is, the material responds to changing magnetic flux by changing its dimensions. While the dimensional changes are small, it is enough to produce sensible levels of sound. I have never experienced this sort of behavior from electrolytic capacitors.
At this point a question remains. Is the regulator circuit associated with these capacitors and inductors doing something irregular that is producing this noise? Or, is the regulator behaving properly, and this noise is simply due to that fact that the CPU current demand is changing (CPU current is very dynamic) and this is causing the sound? I am thinking that the second possibility is most probable. As I don't have a schematic, or a board to examine, I can only guess. But most likely the two inductors and two capacitors are the output of a two-phase buck regulator that reduces the voltage from the external power adapter to something like 1.5 Volts for the CPU. If this is the case, one would expect the variation in CPU supply current to cause the inductors to produce sensible sound. If it is the inductor(s) that is producing the sound, I am not sure what one can do about it. Perhaps some non-conductive material can be placed over the inductors to absorb the sound. Some rubbery stuff that is acoustically lossy might work. One thing that comes to mind is the materials that are used to remove heat from electronic components. This material is sort of gooey and is very soft and conformal, so it can be applied to irregular surfaces. This stuff is referred to as "Thermally Conductive Gap Filler Pads". One supplier of the material can be found here <http://www.chomerics.com/index.html>. And more specifically, the HCS10 material is here <http://vendor.parker.com/Groups/Seal/Divisions/Chomerics/Chomerics%20Product%20Library.nsf/24eb4985905ece34852569580074557a/586cf13e3b4b8ff08525729600612c6c/$FILE/THERM-A-GAP_500_Series_TB-1016-1-07.pdf>. If you are interested in this solution you might call the company and ask about acoustical properties. Usually suppliers will send free samples for evaluation. Notice that the HSS10 material is very soft and is available in up to 0.20 inch thickness. -- Gavrik
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