Maybe its a belt?
I've heard belts squeal before, maybe the belt is squealing until the heat of the engine heats it up?
I'm not a tech so thats just my opinion.
Does it sound like its coming from the front of the car? The rear? that might help the techs here diagnose it better.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.