[DonRotolo]

To convert DC voltages, you turn it into Alternating Current, then use a transformer to change (bring up in the 24 volt case) the voltage, after which you turn it back into Direct Current.

Lower frequency AC requires larger transformers. To maintain efficiency and keep costs down, these circuits typically operate at several kilohertz - that is what you are hearing, the kilohertz frequency acting upon some circuit element.

The whine from the motor when you use a victor at partial power is a similar concept.

Don