If you have 48v phantom power, check to make sure that it is turned off when not needed. Its possible you're picking up "line noise". When recording, it's best to have as few 'linked' electronics on the same circuit [outlet] but, in home studios short of hiring an electrition, thats not possible. Whether this solves your problem or not... don't fret. Thats what NOISE REDUCTION is for. Leave 8-10 seconds of dead air at the beginning of your song with an open mic, sampling the noise in your environment. NOISE REDUCTION within that 8-10 second window will calculate and remove mic and line noise throughout the entire recording.