Sounds like exhaust rattling on the bottom of the car. Grab your tail pipe and give it some good tugs left and right up and down and see if you can get it to bang off the car.
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1999 BMW 328i (Daily Driver) - m54b30 intake manifold, ebay headers, Rebirth Motorsports CAI, Rogue Engineering underive pulleys, Epic Motorsports Remote Dyno Tune, Custom 3 inch exhaust with Burns Merge, Schrick 264/248 Cams
1967 Buick Skylark - 430 Buick Big Block Swap.