two things I immediatly thought of:
heat shield rattle. see if this noise happens at a certain rpm. If so, its likely just a rattle along those lines. nothing to worry about.
throttle cable is frayed. this i would definatly worry about. It doesnt sound like this is your problem, but I'm just throwing it out there. I screwed up my cable when I was changing my alternator. Trying to get that thing out, i damaged the cables plastic housing, allowing the metal cable to fray and catch the "insulation" (i dont know what its called). So one day I ended up with a stuck throttle starting at 50mph going down a hill. On an additional note: don't COMPLETELY kill your car's ignition, or else your steering wheel locks and brake booster turns off...just fyi....
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