I hear like this rattle below my car when the temperature passes 200+, i only hear it when the car is in drive. If I put it in park or neutral and semi rev with the same temp i don't hear the rattle. Yes it's a V8 89 IROC, and I already replaced the muffler to Flowmaster and my catalytic converter is only 3 months old.

 

still might be ur cat, did u try getting under there and banging on it? or getting under the car while in drive, having someone hold brake? maybe its not the cat and just the pipe vibrating next to something.

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- David
88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT

 

Pinging?

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"So many Mustangs, so little time..."
ICON Motorsports

 

is the cat welded or clamped?

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- David
88' GTA 5.7L TPI MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass, Gutted CAT, Flowmaster 80 Series

 

I agree with RB...pinging

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1984 Z28 350
Never argue with an idiot, they'll just bring you down to their level and beat you with experience
My Web Page

 

the cat is clamped but I'm not sure if it's JUST the pipe because it takes me along time to get up. Like i step on the gas and car gains speed very slowly and makes very nasty sound underneath. I'll double check the cat.