I also have this rattling when I hit the gas suddenly. Sounds like a heat shield or something that rattles. It makes a sound like someone taking a couple of marbles and shaking them in a soda can.

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1999 Silver Frost 2.0 SE Zetec
Sony ES Mobile Xplod Stereo
BAT Bright Whites

Kharma is very real and she is one mean b!tch.