I had a similar issue, vibration, not a rattle, on the down rev, right around 2000 RPM down to 1800 RPM was the worse. You would have almost thought it was in the drive train it vibrated so much, mainly as I approached 2000 RPM, in 4th or 5th gear, on the down rev only. I also had a slightly rough feeling idle.
I pulled and cleaned the ICV, with carb cleaner, and the vibration went away. Idle smoothed out too.