In the world or audio you learn new and interesting facts every day.
AC caps (ceramic, electrolytic, polyester and polypropylene metallized foil types) can buzz at various frequencies. This is not nonsense, and is quite normal behavior; it is common sense if you stop to think about it. As the voltage switches from positive to negative in the presence of AC the film stretches and shrinks with each cycle, just as a transformer lamination stretches and shrinks.
Transformers, coils and capacitors are all capable of acting as little door buzzers.
Unfortunately cap manufacturers do not include specifications for audible noise in their technical data, so those of us using AC caps in applications where audible sound levels are critical have to test each type carefully and specify only those that meet our own QC procedures for audible noise.
Interestingly, the tendency to hum is entirely unrelated to the quality, price, or electrical specifications of the cap. So a noisy cap does not indicate a failing or low quality cap at all.
For many of the AC caps in PurePower units we have worked with a small custom manufacturer that has developed a unique folding configuration to reduce the tendancy to vibrate. Several of the hand wound audio grade cap manufacturers have worked very carefully to develop quiet caps through trial and error to come up with the ideal winding technique, and one California audio grade custom cap manufacturer told me it is a skill that varies from employee to employee.
Usually, the sound produced by noisy caps is below a level that is audible outside the component case, but occasionally they can be loud enough to be objectionable.
p.s. It would not surprise me at all if there were capacitors happily humming away inside some very high end audio equipment. Fortunately, the sound level is usually proportional to load, so they would only make noise when the music is loud, and catching them would be like trying to see if the fridge light goes out when you close the door.
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