Got a Sound Ordnance M-4075 amp for my 2008 F350 6.4L Diesel regular cab.  Head unit is a Kenwood KDC-HD545U.   Amp is being run three channels.  Front two for loud speakers rear two bridged to a sub.  No other speakers.  After install I had a brutal buzzing from the speakers with the ignition in the RUN position.  Running I also got an alternator noise but I'll deal with that later. 

So I've narrowed down the buzzing to an issue likely with the head unit and the RCA grounding.  So I soldered a grounding wire to one of the front RCA's at the back of the HU.  It's grounded with a 3amp fuse to the HU body.  BAM!  No buzzing what so ever with the front RCA's plugged in.  But when I plug the rears in for the sub it comes back.  Not nearly as loud as before but definately noticeable.  I tried grounding those RCA's as well but it didn't do anything.  So I'm kind of scratching my head at this point.

One other piece of info.  If I gently plug the subs RCA's in to the HU just shy of the grounding shield (not sure what you call it) touching the female connection outer shell grounding shield (term?) there is no buzzing sound.  But as soon as that grounding shield of the RCA's touch the outer shild on the back of the HU the buzzing is very apparent. 

So I'm halfway there to eleminating this buzzing (except the alternator noise) but I'm stumped.  Any help would be appreciated.

Stupid me.  Solved.  I never actually checked the rear (sub) RCA's and their location to other wires.  Turns out that was the source of the rest of the buzzing.  I moved both sets down the middle of the truck and no more buzzing.  Nice clean sound.  No buzzing when running either.  So I'm all good.  Maybe this can help someone in the future. 

Oh.  The soldering of a ground wire was still necessary to aleviate much of the noise.  So I'm assuming something went bad in the HU?  Does that sound right?

http://bcae1.com/images/rca/temporaryrcashieldrepair.html