I hear a humming noise from left side of engine compartment for 1 to 5 minutes after I shut down my 2015 ML 400. Any insight?
Like the comments have already mentioned, I do believe this is an issue with the cooling fan.
Cooling fans can be extremely loud and what you are talking about is probably the same thing. However, I have never experienced this issue myself. It's better to get it checked asap, it might sound like a small thing right now but can turn into something very serious.
I suppose it has LED lights. They have cooling fans, which continue working several minutes after you turn off the car.
Does it sound like that?
Good suggestion Boki.
It could be the cooling fan as well.
Have you opened the hood and tried to pinpoint the exact location?