Dnasty

Here is the link https://voca.ro/1f7hdkCqhnHy

My car has been making this noise for at least a year now when I turn it off and sit in it (it stops once I open the door). I am also having intermittent long cranking issues occasionally and am wondering if this noise is related. Possibly this is the fuel pump or the amp as someone had suggested in another post. It seems to be coming from the rear driver side. You may need headphones to hear this. Thanks
fyi. It is the long humming noise that goes in and out three times on the clip.

Chevota

I had a 2000 ML55 that had a similar weird sound, but it's been too long to for sure recall the sound or when it did it but I think it matches. Anyway, on the drivers side under the hood, closest to the side mirror there was big black plastic box that was strangely rather deep. I forget what it was for, maybe the fuse box, all I recall is was deep and at the very bottom was a little fan. The fan had some classic fan noise issue, like it needed lube or something was touching the blades to make noise and driving me nuts.
Obviously not your model but these cars have fans and little motors here n there that do who knows what. It took me a while to find my special noise, so you may have to do the same and try each day until you narrow it down.
Good luck