ground, clean all antanea mounts, cable connections, etc... easiest to do before spending money on noise filters that don't do anything IMO. Cleaning grounds has always improved back-ground noise for me.
Yep even with DSP filtering the best way is to kill the noise at the origin.
EVEN a cheap CB can hear a .000,000,5 volt signal ( .5 UV ) and it dosn't take much for a motor brush or spark plug to make a bunch more than that.
POWER cord filters are useless for 99% of radios unless your electrical systems is totaly screwed up ....