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