My S550 A10 has always had a wine in 1st gear but not in neutral or reverse.I'm having the same issue as everyone else it seems. Just picked up the car yesterday and it's only 100miles on it. It doesn't seem to be there when I first start the car, but like others have stated, once the car warms up it's definitely there. Mine isn't so bad that I can hear it while driving or even at low speed with windows up, but soon as I wind the window down or open the door with the engine running, I can here it. With the hood up and standing right in front of the engine, it's hard to hear but when I come down the right hand side (drivers side in the UK - RHD) I can hear it. I can here it all the way down the side and then if i put my head under the car at the rear, I can hear it coming from somewhere ahead of me. Happens in park, neutral and drive and does increase with revs. Mines a MY24 V8 GT Auto.
I'm glad I'm not the only one having this, but does seem to be mixed resonses between auto box, serpatine belts and other suggestions. Must admit it does sound vagulely like power steering but also not. And if it were power steering it would change sounds as you turned the wheels which mine doesn't. It's not unbearable but like most people, just want to know it's 'normal' and not cause for concern!