My '99 Toyota Corolla with 145K miles is making this rattling noise. It is audible in drive and reverse, but not in neutral or park. At first I thought coming from the dashboard area, like under the hood. However, I tried listening under the hood and couldn’t really hear it over the engine noise. Finally I realized it actually seems to be coming from under the dashboard on the driver’s side, right above the pedals. I made a recording holding the microphone down there. Here is a link to the audio file: http://dl.dropbox.com/u/313357/recording.mp3
Have you tried turing blower On/ Off? I was wondering if the ac/heat blower is making sound.
Yes. That’s actually making a totally separate noise - a high-pitched whistling noise from the passenger side vent. But no, this rattling noise is not affected at all by the blower setting. It was off during that recording.
Yup. Someone put an Ace of Spades in your vent, I know that rattle anywhere. Just kidding. 
In all seriousness, have you looked under the dash. There’s alot of plastic kick panels down there that could rattle. That’s a great way to get the sound for us to hear, modern technology boggles my mind! I’m making that sound my ringtone. JK, again!
Hopefully, Caddyman and some others will be along shortly. There’s some serious brain power in this chat forum.
Hope you get it figured out, it would drive me nuts! Turn up your radio in the meantime.