are you using auto focus? Normally, people use external audio recording and sync it with the video in post because the camera will pick up noise from auto focus and/or when you are adjusting the camera. Also built-in mic is mono so the sound quality is not good at all.
HI, could you specify more clearly what kind of sound does it pick up?
Any AF noise will likely be picked up by the mic/audio recorder. Disable AF for serious video shooting if you can't place the mic far from the camera.
TIPS FOR AUDIO RECORDING WITH EXTERNAL DEVICE connected to the camera:
This is possible thanks to the latest firmware 2.x for the 7D that allows to select Manual Audio Gain control. It's also possible in every other camera with Manual Audio Gain like 5D2, 5D3, 60D, T3i, etc.
- Disable Auto Gain Control. Select Manual Audio gain instead.
- Move the gain slider all way to the left (to zero) and then move it only one click to the right (this will enable audio gain, but to its minimum)
- Connect the mic/recording device to the camera via male to male stereo cable (usually shipped with the audio recorder) this way: "Line Out/Headphones" (in the audio recorder) to the "Mic input" (in the camera).
- Set the Audio Gain / Headphones Volume in the recorder (checking the audio levels on the LCD of your camera) to reach about -12db as average (with the sound source you will record). Avoid clipping audio, and setting it too low.
- Using this method you will get audio on your footage with LOT better quality than using the buil-in mic of the camera and the audio will be already synchronized with the video.
- If you want to record audio with the audio recorder, you can, so you will have a second audio file (called "double audio recording") of even much higher quality (if you set the recorder accordingly) that you can easily sync in post to replace the audio track recorded by the camera.
NOTES & SUGGESTIONS:
- If you use a mono Mic/Audio recorder, you will get mono audio in your footage (obviously)
- Place the external Mic/Audio recorder as near as possible to the sound source, and as far as possible from the camera. If you can't put the mic/audio recorder out of your rig, then put it as far as possible from the camera and lens (and not behind it). In this case the best position is in front but at a side.
- Good Audio recorders that have extremely versatile features are Zoom H2n and Zoom H4n. You can record audio as mentioned above with both of them using just the buil-in mics of these recorders. Just be sure to adjust them right.
- Always do some tests before shooting. Fixing bad audio is very difficult, sometimes more difficult than fixing video.
- Always use some kind of wind screen over the mic/audio recorder, otherwise you'll get noise even from a slight breeze or when moving your camera setup.
Hope this helps!