try this:
turn the car/radio on but mute the volume. now wiggle or bump the rca's to see if you get the same "bumping" sound through your sub. if so then you need to re-route the rca's or simply replace them if they are bad.

if its not your rca's, then check your amp's ground. needs to be to bear metal and should be tight against the chassis (unable to move around by pulling on it).

so having issues? at this point I would place my bet on legacy. They have a known reputation for noise issues with their amps.