The CD-2’s professional quality internal microphones are quite sensitive and are omni-directional--meaning that their microphones absorb sound from all directions. This can occasionally be less than ideal if you are recording in an environment that has any constant noise; even if the noise is relatively low in volume. A suggestion would be to move the CD-2 as close as possible to the source of audio that you are recording (e.g. an instrument, the person singing, etc). Be sure that the CD-2 is not placed on top of a surface that may be contributing to the noise--due to vibrations, etc.
Another suggestion would be to use a good quality cardioid style microphone (not omni-directional). Cardioid microphones accept audio in front of the mic and reject noise around and behind the mic.
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If you are recording a singing voice, a condenser microphone is commonly used. For example:
Condenser Microphone
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If you are recording dialogue, a dynamic microphone is commonly used. For example:
Dynamic Microphone