Strange Grinding/Grating Noise @ 1500 RPM

[Amac]

You hear it whether the car is sitting in park or is in drive and moving down the road so I'm thinking engine vs transmission. Any ideas what might be causing this? It is fairly loud, you can hear it appear just below 1500 RPM and it disappears just above that range.

It runs fine at highway speeds and higher RPMs.

It is a 2002 QX4 with ~ 102,000 miles.

[Pwnin O'Brien]

Sounds maybe like an exhaust heat shield, it's extremely common and can be heard inside the vehicle and outside the vehicle. I'm having the same exact problem with my car right now.

[SixGuns]

Exaust shielding. Crawl under the rig with the engine on, pinpoint the rattle, bend up the metal off of whatever it's rattling on. Sometimes impossible, as it's the actual pipe itself it's rattling on. Sometimes you can get around it.