I am thinking you should to check the adjustment bolt on the hub/disc, because my left front one was the squealing one, and that disc was very loose, like it could have come off the car. It doesnt take too long, just pop off the wheel, and grab the allen wrench and check that bolt for looseness.
I only noticed it when I was changing the pads because it moved back and forth alot, and the pads were unevenly worn, the sides were a little lower than the center, which tells me the disc was wobbling perhaps.
My brakes feel great now.
Definately check the DIY Article on E-class brake jobs, it will show you the adjustment bolt.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.