I'm not an expert in audio but in 'professional' mic's, they have XLR connectors and if I understand this correctly, the X is for earth, the L is for left channel and the R is for right channel? What you seem to be picking up is a sound at 50hz, now correct me if I'm wrong anyone, and that is called mains hum. Maybe if you could 'earth' your mic some way it might reduce it or get rid of it, it can also be gotten rid of in post production as it is a recognised sound with a recognised frequency. Most (good) audio and video editing app's have filters to get rid of that sound.

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