Whenever I'm idle at a stop light or something and the RPM is below around 900, the exhaust rattles somewhere right below the passenger door. My dad said his 86 IROC 305 did the same thing. It's annoying...especially when everyone around me in their cars are staring at me and thinking my car is a POS.

 

Sounds to me like your catalytic converter may be clogged. But it could just be a loose bolt or something holding up the exhaust there. Does the exahust usually sound like it flows freely, or does it have a kinda blocked sound to it? I suspect the cat though.

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1986 WS6-Equipped 5.0 TPI Trans Am
Random Tech High-Flow Cat
Two Chamber Flowmaster Muffler
Megs Custom 4.5" Stainless Steel Rolled Tips
1999 Grand Am GT In-Dash CD Player w/Equalizer(fits like '86 original)
5% Tint On Rear, 35% On Sides
http://www.pitt.edu/~zyhst/trans86/

OTHER RIDE:
1998 C5 6Spd. Corvette Coupe
Silver w/Gray Leather Int.
12/14 Options
Stock(for now)
Best 1/4th: 13.5 @ 104.4MPH

 

There is a Heat Shroud right above the cat. It keeps all that heat out of the interior. Cats get around 1200-1600*F. Get under there and look pull on it and you'l be able to tell right away whether its loose or not.

-Lars

 

I am having the same EXACT problem. I thought it was the heat sheild also, so i pulled it off... didn't help. It seems like ther is something rattling IN the cat. I think that maybe a peice of the honey comb has broken off and it rattling around. I am gloing to disconnect it and find out.

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• 89 Formula 305 WS6
  SFC's, LCA's, Air Foil
  Spohn Panhard Bar
  Flowmaster Cat Back, Tranny Cooler
  A/F Ratio Meter (not a mod but it sure spurs interest)
  Trans Temp Gauge
  Most "Free" Mods
  3:73, Zexel Torsion Posi
• 90 Eagle Talon 2.0
  (Winter Beater)
  Mods? Um, no

 

I used to have the exact same problem.... my cats were no good on my dual catalyst 89 IROC-Z. Swapped them out for new ones and no more rattle!

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1989 IROC-Z
350c.i. TPI
rebuilt 700R4 with shift kit
3'' Monza Pacesetter cat back exhaust (minus the muffler)[SOUNDS OF THIS ON MY WEBPAGE]
new dual cats
3.23 Posi rear
hypertech chip
under drive pulleys
adjustable fuel pressure regulator
K&N Air Filters
Excalibur alarm, keyless entry and remote start
new:
struts, shocks, bearings, distributor, cap, rotor, alternator, battery, regulator
245 ZR16 50 Tires, Rotors and brake pads all the way around

Go to my webpage to find out more about it.
You can also see pictures, and hear sounds as well.
http://go.to/iroc-z

AOL IMer: JRShotliff

 

When your Cat fails, it may not clog up to the point that you loose power. Usually the catalyst in them melts into chuncks that rattle around inside the can.

 

I took a look at mine last night. The car was cold and i had already torn off the sheilding on the cat. I smacked it a couple of times with my hand and i could hear all the broken peices rattle around. Needless to say but, i took all of the "broken pieces" out... i don't like the sound though. Time for a random cat!

 

Omega...

Rest assured that if you get a Random Tech Cat, you wont be dissapointed in sound or performance! Take it from me...one word....LOUD

------------------
1986 WS6-Equipped 5.0 TPI Trans Am
Random Tech High-Flow Cat
Two Chamber Flowmaster Muffler
Megs Custom 4.5" Stainless Steel Rolled Tips
1999 Grand Am GT In-Dash CD Player w/Equalizer(fits like '86 original)
5% Tint On Rear, 35% On Sides
http://www.pitt.edu/~zyhst/trans86/

OTHER RIDE:
1998 C5 6Spd. Corvette Coupe
Silver w/Gray Leather Int.
12/14 Options
Stock(for now)
Best 1/4th: 13.5 @ 104.4MPH

 

I had the same problem. On mine, there's a little steel line that runs from the cat to somewhere else and under the passenger seat, I stuck a piece of wire between it and the exhaust pipe and it stoped rattling. A ghetto fix, but it worked.