Is the noise from the ignition or the generator/alternator?. Have you tried disconnecting the generator/alternator and see what happens? Even heater motors can produce the same noise at times from the brushes. capactors on the output of a generator/alternator can get rid of most of that type of noise or adding a choke. May also check where the amp is getting it's power from, may have to run a wire off the battery throught a fuse may also help, can even add a relay to the acc terminal to torn off the amp with the truck. Ignition noise is another problem all together, check the cap and rotor for condition, and the type of plugs you are using may make a difference. See if you can fing a copy of the Radio Amateur's Handbook publish by the ARRL ( American Radio Relay League) quite a few of our public libraries have copys and have lots of information on noise problems with radios.