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Blame the constant reversing of the AC current. That switching back and forth excites the core and case, and makes them want to boogie on down. The hum is composed of harmonic frequencies.In the U.S. the reversal happens 60 times per second (referred to as 60Hz - or 60 Hertz). This frequency and it's harmonics of 120Hz, 240Hz, are close to the musical note Bb. The 180Hz harmonic is close to F#.F# is the augmented 5th of the Bb major scale. However, Bb is the natural 3rd in the F# major scale. This would imply that U.S. transformers like the key of F#. That is a key that was favoured by Franz Liszt."Mozart & Liszt" is Cockney slang for "drunk". "Drunk" rhymes with "clunk" which is a sound often heard when a relay is activated. Relay activation can be caused when a transformer is switched on. So there we have it - a connection between transformers, a Hungarian composer and Cockneys. I bet you never thought that could happen! 
Yes the reason that it hums is that a transformer is basically two electromagnets wrapped around a piece of iron. When a current flows one will induce a current in the opposite direction in the other, this will mean the two electromagnets will attract one another and any bits of iron in the device. As the alternating current drops to zero this force will go to zero, then as it increases in the other direction both will start to attract one another again. So the transformer will be vibrated at twice the mains frequency, and probably depending on the load it will have some components at the mains frequency and other multiples of it.
As stated above the hum is produced by the contracting and expansion of the iron core due to the changing AC wave. The iron core of a transformer is not just one chunk of iron and is usually made up of thin laminations of iron or steel clamped together. This make thing cheaper, helps stop eddy currents and lessens the hum produced from the supply freq. You may notice this hum easier on cheaper or older transformers as their laminations may not be held as tight making for greater vibration.Toroidal transformers however are more efficient and produce very little hum as they are made from a single strip of steel with the coils tightly wound.
Is your dam anywhere near the outlet pipe of a rehab clinic ?
Quote from: neilep on 14/02/2008 00:13:14Is your dam anywhere near the outlet pipe of a rehab clinic ?No. It's in Amy Whitehouse's garden.