2003 Civic EX, 58k miles

I'm hearing a rattling sound when my engine is at 1,000 rpm. It happens whether the car is in park or in gear, as soon as the engine hits 1k rpm there is a short rattling sound that stops after the engine passes ~1.1k rpm. I don't feel any vibrations through the steering wheel or the gas pedal though. From inside it sounds like the noise is coming from the passenger side, but outside with the hood up the sound seems be more centrally located. I can barely hear it at all from outside the car with the hood closed. Can't find anything that is obviously loose, and haven't noticed any drop in performance, it runs fine at all speeds.
Anyone have any thoughts on what might be causing this noise?
Thanks!

 

I'd say the heatshield around your catalytic converter could be the most likely of culprits.

 

Thats what i was gonna say. Mine did the same thing and I jacked the car up and there it was. The heat sheild was loose and making a horrible noise but only at around 1000-1100 rpm.