Make sure the line input is pushed all the way in. Thats the easy fix. If it is, it could be power hum. Make sure the line input cable is away from the power cord of the sub and other equipment.
I had mine do that the other day. It was becuase of the teporary sub cable I rigged up using two RCA audio cables spliced together with a RCA butt connector. Changed the cables out for a single long run and...no humming.