Our new GL has a loud resonant hum that occurs usually between 1500 and 2400 RPM's or so. It's a fairly high-pitched and sounds like something is vibrating. At first I thought it was a whistle but there's a vibration to it that makes me think that something is loose (or touching something it shouldn't) and that I'm hearing whatever it is resonate.

A couple of service dept. visits hasn't turned up anything. It doesn't help that it's hard to reproduce sometimes, especially if the vehicle or the weather are cold. The service tech was thinking it was a loose exhaust shroud or similar.

Here's a 2 minute video showing a few occurrences (it's best to use headphones to hear it):

Any thoughts or ideas about what this is?

 

Is this your first turbo'd vehicle?
Tough to say from video, but I'd guess you are hearing the wage gates bleeding off boost.
I find it much more apparent when the this vehicle is cold.

 

It's my 4th (although the previous 3 were all Bluetec models).

I initially thought it could be the waste-gates as well, but it's loud (especially if I'm next to a construction barrier), and it has a vibrato / rattle quality to it.

 

It's not an unheard of complaint with this car. Most describe it as whistle that presents itself at a certain rpm range. I've heard this whistle myself and have just written it off as a normal exhaust/turbo pressure noise.

Vibration or rattle could be something entirely different though. Keep us posted. Good luck.