This document describes an issue where the Apple DVI to ADC Adapter produces a low volume high-pitch noise which can be heard.
Symptom
When the Apple DVI to ADC adapter is plugged into an AC outlet and the monitor is in sleep mode or off, you may hear a low-level, high-pitch noise coming from the adapter.
Solution
It is normal for you to hear the high-pitch noise when the monitor is in sleep mode or shut down. However, if this high-pitch noise is heard after the computer starts-up, or after waking from sleep, contact Apple support.