Thanks for the info on the shifter....I figured it was something like that. I can pretty much make the other rattle occur at will. All I need to do is accelerate (not even particularly hard) when the car is already rolling. The weird thing is, if I rev the engine when the car is parked there's no noise, so if it's an engine noise the car needs to be under load. I don't think it's a tranny thing, it sounds like it's coming from the front of the car. I'm still leaning toward a belt tensioner or something along those lines.....
--Ethan