I noticed this the other day when driving home from work. After the car was hot, it started making a quiet rattling noise from behind the firewall around the center console when in gear and coasting. Didn't seem to be speed or RPM dependent, nor did it matter what gear I was in. Disappears after going below about 25mph. I checked both fans for debris, but didn't see any.
Didn't notice it when starting up the car, but once warmed up, the sound returned. What could this be?