Have you listened to the microphone input to verify there is no noise when DCS has focus?
If you mute DCS in the Volume Mixer, does this still happen?
To do the former, right-click the speaker icon in the system tray, click "Sounds" in the context menu, click the "Recording" tab, double-click your microphone, click the "Listen" tab, check "Listen to this device", click "OK" or "Apply"