I was looking for an answer on the forum but I have a 2000 SL 500, and there is a loud whine that is constant when the car is running. The sound is loudest in the rear, and I’m not sure what it could be, since my SL isn’t equipped with air suspension or anything like that. I attached a short vid with the noise.

note: we recently replaced the fuel pump (was not an oem part)

 

Get a mechanics stethoscope at Harbor Freight and crawl under right rear of car and listen to the pump

 

If the noise started after you put a non OE fuel pump on the car, then the problem is obvious...

Which brand did you use, lol.. Should've only used Pierburg

 

It sounds like the fuel pump.