2007 GMC Yukon XL 2500 Radio Whine

I just purchased a 2007 Yukon XL 2500 with a Factory DVD. It has a whine in
the speakers all throughout the car. It happens if the radio is on or if the
blinkers are on. I know the blinkers run through the system. The sound occurs
whether the truck is running or just in auxiliary. I had a professional audio shop
go through it and all he could think to do was change the amp. (No luck) He
said the next step would be to replace the head unit, but he's not even sure
that would fix the problem and that's a lot of money to spend if I don't have to.

I have removed the rear audio controller and the DVD screen in hopes that it
may be there. (Saw elsewhere that rear audio control could be it). Also, no
luck.

I am at a loss for what else to do. PLEASE HELP!!