Another problem could be that you ground is not secured properly (ie the surface could have paint or some other form of debrie allowing an improper ground to your amp) another could be if you are running a single loop setup ( all negitive leads going back to the negitive post on the battery or one location for all grounds on the chassy of the vehicle ) not a good idea ensure all grounds are seperate. To fix this ......if you have already tried a noise filter......is to run a stiffening capacitor, now i know these are for easing the jolted loads on the battery but they also work very well to get rid of noise from the alternator......a good rule of thumb for this is 1000 micro ferods per watt of you system at about 16 to 20 volts