i'm not sure if this is serious or not, but it just started:

- start the car, a rattling noise can be heard when sitting inside the car, when idling, coming from around the front passenger wheel
- outside the car, i can't hear much of anything
- when the car is still but revved, the rattling noise (again from sitting inside) goes away
- when the car is dropped back down to idle, the noise comes back

i don't know anything about the car's structure (in terms of mounts or anything), but does this sound like anything to worry about?

Sounds to me you may need youre timing chain and tensioner replaced. I know on my 01 cavalier had to get it done, Due to the noise on my passenger side.

Good Luck.