My 2005 Mercedes C240 4matic wagon (126,000 mi) has had an engine whine for as long as I can remember. Any thoughts?
Your engine is unlikely to be making the noise. The usual suspects are power steering, transmission, and differential. If you have awd I’d start with the rear differential but a good mechanic with a stethoscope can pinpoint the noise if you really want to know.
You sure it’s the engine?
If so, it could be an idler pulley
The others have good suggestions. Also, if you turn the radio off, does it still do it?
Is the noise engine speed related or vehicle speed related? Take the car out on a road when it is not crowded and downshift to see if the noise changes pitch. If you can do it safely try putting it in neutral when moving to see if it changes.
Does it do it when the car is not moving?