I ran into an interesting problem a few weeks ago so I am sharing in case it helps anyone else. A client in a large commercial building asked for help because a large transformer has started vibrating and making a loud buzzing noise. I used a Fluke 43B power quality analyzer to measure the harmonic spectrum of the secondary current. The spectrum had 2nd, 4th and 6th harmonics, indicating the presence of DC current in the transformer secondary winding. DC current tends to saturate the transformer core at the peak of one half of the AC waveform. As the core goes in and out of saturation, it vibrates and causes the buzzing noise. I wondered if the source of the DC current might ben an input semiconductors of a motor drive that had failed and so was providing DC current. I checked around and found that one large drive was operating a ventilation fan. I turned off the drive and -- bingo -- the transformer stopped vibrating.