First check all wiring.. especially your sub-bass amp's gound.. the ground has to screwed onto bare metal.. no paint whatsoever.

The ground wire should be no more than 8-9inches from the amp if possible. The ground location must be car chassis metal.. not to a seat bracket, etc.

Next thing after ground is RCAs and power wire.. they must be seperated by at least 12 inchs or so... if they must cross have them cross " _|_ " <--perpendicular to one another.

Try moving your ground around..this should clear up the noise... I tend to stay away from ground loop isolaters.

So issues to check... (1) ground wire location, connection, length (2) RCAs.. shielded?, bad quality..worn? (3) RCA and power wire prximity... keep far apart as possible..