Make sure your grounds are to a good point and not just against painted metal. Needs to be bare metal. And I usually try to ground within 18" of amp. As mentioned before making sure the signal and power wires are separate by I would say 12 inches

Are they shielded RCAs ? But 95% of the time I've found it's not a proper ground. And if not that then a faulty signal or bad equipment internally

 

that can happen if your rca cable runs next to your rpm gauge wire. they sell a filter for it that plugs into you stereo and its like 30 bucks i believe. my brother had it happen in his tahoe

 

I'd like to thank everyone who mentioned "bad ground wire". That was my issue, and here is why... I used a factory body bolt and nut to attach my amp's ground wire. I hit them with a scotch pad / steel wool pad, but they are both painted with thick primer. Scuffing them up allowed them to pass enough current to get the amp to work, but not enough to avoid alternator whine.
I bought a set of these to go on the end of my drill:

It still took a few minutes to reach shiny metal, but once all of the contact area on the body, bolt, and nut where down to bare metal... no more alternator whine!

So... thanks!!!