Engine Compartment Rattle

[ChiliPepperSC2]

My car has normal rattles, but this is a new one for me that I'd like to diagnose. It started about two weeks ago when the nights were getting into the 30s and 40s. When I start up my car in the morning, I hear the rattle. It's similar to my ABS rattle (I call that the "marbles in a can" rattle), but it doesn't last as long and it's more subtle. My car is a manual, and when I accelerate out of my parking spot I hear the rattle again. When I shift into 2nd/3rd/4th/5th, I hear the rattle. I did an experiment one morning, and if I coast along and engage the clutch just a few inches then disengage fully, I can hear the rattle. I did this a few times in a row, and it rattled each time. If I accelerate suddently, I hear the rattle. Once my car is warm, the rattle goes away. If my car sits and cools down and it's a cool day, it'll produce the rattle again. The source of the noise seems to be from the engine compartment's passenger side near the floor board, but since I don't carry a passenger often, I haven't had the the opportunity for someone to sit there and really listen for me.

I had my car in for an oil and filter change the other day, and the mechanic wasn't able to find anything and didn't hear anything. I'm sure my ears are much more in tune to my car, because as I pulled out of the parking lot, it rattled. He checked my TAMS and said everything looked fine. Thoughts?