This is usually caused by the function of "Automatic Gain Control" or "AGC" on the camera.
Basically, when the camera cannot sense an audio input signal it automatically increases the gain of the input. It is increased so much that the noise floor in the camera microphone amplifier becomes audible .
The AGC can be disabled on some cameras and it is worth checking with you camera manufacturer for information on this, which can sometimes require a firmware update to the camera.
RØDE have created the VideoMic Pro to help combat this issue, the VideoMic Pro contains a 20dB pre amplifier, boosting the mic signal so that the camera will see a good signal level and not engage its AGC.
We recommend setting your audio levels manually to achieve an optimum signal to noise ratio and disabling the AGC of the camera.