If you have cross overs (which in a 6x9 case you wont) if you have them installed near ur ECU that will cause motor feedback, to solve this, make sure all termincals are insulated, and wrap the cross ove rin foil, then with a peice of wire, wrap it around the cross overs, then to an earth point.
Wires running past the ECU could also cause this noise if they are crap insulation/guage