Keep in mind that you could encounter a buzzing or humming transformer those PowerVar or Oneac units depending on their condition and also if there's any DC present in your power delivery.
Here a link to an excellent paper explaining the conditions that cause transformers to become noisy:
http://www.plitron.com/PDF/AES.PDF
Here a link to an excellent paper explaining the conditions that cause transformers to become noisy:
http://www.plitron.com/PDF/AES.PDF