The Rode VideoMic is awesome and a very nice microphone for the price. The problems with the loud hissing noise came through from the 5DM2 and it's AGC (automatic gain control)
The audio on the 5D was driving me crazy. A while ago I purchased the Rode VideoMic in hopes to improve the audio on the 5D but the biggest problem was really the AGC ( auto gain control ) constantly boosting the sound and creating this annoying hissing noise when the room was silent. No matter what setting I used on the Microphone it seemed the 5DM2 would always override the settings.
Discovered Magic Lantern firmware that is supposed to disable the AGC. This thing really really works. You have to set different gain levels when using the firmware depending on how your microphone is setup, but what's cool about this firmware is that it allows you to attach headphones to the Audio/Video port on the 5D so that you can monitor the audio!!! In my short little tests the Audio was 100 times better. Awesome.... Here's something for you guys to listen to (excuse the video quality, it's about comparing the sound).
I basically went insane about this problem, so I decided to test out the firmware. DO NOT TRY THE FIRMWARE IF YOU ARE WORRIED ABOUT YOUR CAMERA. THIS IS UNSUPPORTED FIRMWARE THAT COULD DAMAGE YOUR CAMERA. Although I haven't run into any problems with the firmware, these have always been very short (successful) tests. Each time you reboot your camera you will need to reload the firmware (that's a good thing). Just for kicks, I always pull the battery out after powering down to make sure it's really out of memory.
Rode VideoMic set to 0db
8' extension cable
Canon 5D Mark II with and without Magic Lantern Firmware