I connected a single phase step-down transformer in step down configuration and found that a certain amount of humming was audible. But when I connected that same transformer in step-up configuration, the humming increased largely. Also smoke started emanating from the transformer.
It is due to the Laminated Magnetic core not being secured tight.Eddy Currents pass through the Steel strips and a Current carrying strip conductors placed in a Magnetic Field experience force on them and cause vibration of the strips,which in turn cause the Humming Noise.
This phenomenon is also called Magnetic Hum.
P.S.
What you hear is twice the mains frequency ( 100Hz or 120Hz depending on country)
It is because the iron core is moving /vibrating. It depends on the flowing current that depends on self inductivity. The inductivity of a coil / a winding ist proportional to the square of number of turns. ( Voltages are proportional to number of turns.) In case number of turns is too low, self inductance is too low and current going in the transformer without any load is then to high causing smoke / heating of the copper windings.
It is due to the Laminated Magnetic core not being secured tight.Eddy Currents pass through the Steel strips and a Current carrying strip conductors placed in a Magnetic Field experience force on them and cause vibration of the strips,which in turn cause the Humming Noise.
This phenomenon is also called Magnetic Hum.
P.S.
Adding to what colleagues above said, the windings of the transformer also vibrate though they are dipped in varnish due to the alternating magnetic field. You can simply check this by energizing a suitable coil (no magnetic core) using AC source. The vibration occurs at the double the line frequency causes the audible hum at 100/120Hz. This is because the magnetic field reaches its peak twice a cycle. Magnetic hum of either the transformer strips or the windings is also contributed by audible harmonic content at frequencies that are double of the current harmonic frequencies present, i.e. an audio at 300Hz for a third power line harmonic (150Hz). For the case when the transformer configuration was changed to stepping up it is unlikely that what you described will happen unless you exceeded the nominal values of the transformer, e.g. to connect the secondary to a higher voltage than its nominal value or draw excessive current from the primary in the step up configuration. In such cases higher currents will run causing higher magnetic fields and higher vibration and audio and possibility of transformer heating up and smoking. Thanks. @AlDmour.
I agree because the coil conductors also exhibit force,being in Magnetic Field.As explained by Prof. Ismat Aldmour, the vibration occurs at the double the line frequency causes the audible hum at 100/120Hz. This is because the magnetic field reaches its peak twice a cycle,,
P.S.
You have written that when you connected the transformer in step up mode humming was large and smoke was observed but you have not specified the rated voltages in step up and step down configuration and what voltage was applied across the winding which is extremely important. Say you have a transformer with primary of 230 Volts and Secondary as 100 Volts. If you apply 230 Volts in Primary ie winding rated for 230 Volts you will get 100 Volts at output using the transformer in step down mode without any problem and may have little audible noise. On the contrary to use the transformer in step up mode if you apply 230 Volts across the 100 Volts winding the transformer will work with very high flux density resulting the magnetic saturation of transformer core and high inrush of no load current. This if kept for a while will definitely generate lot of heat in the transformer winding and humming will also shoot.
Audible frequency range is 20 Hz to 20,000 Hz. Anytime you have an electronic circuit component switching and/or operating at an AUDIBLE frequency, you have the danger of being audible. Old-style transformers operate at 60Hz (50 Hz in Europe). 60Hz noise is generally called HUMMING. The same problem exists if your switching power supply has components operating at low (say, 5 or 10 KHz).
The frequency matters as much as the amplitude. To prevent humming, you might have to attach (solder) the device better to the PCB or the housing of the case. That way the problem won't worsen by making the device VIBRATE, which amplifies the amplitude. This won't eliminate the audible noise, but will attenuate it. Also note, a device that is switching at, say, 60 Hz will create harmonics at 120Hz, 180Hz, etc ... But, clearly the first harmonic (60 Hz) will be the strongest
Another related topic is: When a cooling FAN is running inside a computer, you hear a noise, which is at the frequency of the turning speed of the fan (say, 3000 rpm, which is 50 turns per second, i.e., 50 Hz, the same speed as your humming noise of the transformer).
It is incaused by stray magnetic fields causing the enclosure and accessories to vibrate. Magnetostriction is a second source of vibration, in which the core iron changes shape minutely when exposed to magnetic fields.
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