We're having an odd problem with our new to us 1999 ML320 (108k miles.) The vehicle just started making this very odd mechanical sounding high pitch noise, a loud Hmmmmm or buzz. You hear it when the vehicle is running but only outside. If you're in the vehicle driving you don't hear it. Any suggestions?

 

check the ps fluid/pump and idler pulley

 

By the left side rear, yes? For me, this sound was my fuel filter.

As an added bonus, your car (and mine - 2000 ml 430) was old enough to qualify for a total fuel filter refitting because they don't make the old kind anymore.

Then, the new one squealed too, got it replaced (under part warranty). That one squeals (in hot weather) to this day, so I just said f it.

This happened to me in Aug 2005, and everything seems ok. I don't recommend saying f it, but at least I replaced the old one.