Sounds like your gasket between the exhaust manifold and H-pipe blew out. Do you have an aftermarket H-pipe? Mac? My GT did the same noise. It was the cheap fiberglass kind of gasket that came with my Pro-Chamber, cracked in two. It'd be worth checking.
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T-Bones 1997 Cobra
Wht/blk, Pro 5.0, 4.10's, aluminum driveshaft, FRPP throttle body, C&L MAF, K&N, off-road H, Flowmasters, s/f connectors, FRPP pro springs, BBK caster/cambers.