I'm starting to think it's a heatshield rattling, maybe one underneath the motor where the exhaust piping is.. my buddy mentioned this kind of problem before on his car.. he put an extra clamp to hold the heatshield in place or together.. I can't remember exactly.

while I was driving today.. I noticed that when the car was cold, or just started driving, there was noise at all.. but once the car's warm the noise came back.. so maybe the heatshield expanded and caused some vibration between the bolts or something..

Here's a pic of my buddy's heatshield, he used a clamp to hold the top and bottom heatshield together..

This sound familiar to anyone? I'll take a look at my car once I get the chance to.