Give that new sub a chance to "break in". New speakers require a few hours of use before the voice coil(s) and spider break in and began operating at their best performance. Is the box within the operating specs of the sub? Is this a ported box; if so you will here some turbulance type noise from the port. What type of distortion are you hearing? Alternator noise(dentist drill like sound) would only be noticable through a sub at lower volumes due to the frequency response of a sub. Is the amps lowpass crossover turned on?
Ok, i have a xplod 1000w amp,with a single kicker cvt 12, and at low, very low volume you hear a low sound of distortion, this a new sub, and the old did not do this, at high volumes,the noise goes away and sounds good, this is only at low sounds, any advice would be appreciated