I have jdm4-1 header/carsound cat. Previosly I had a rattle issue with the cat (heatshield) - 2 metal hose clamps took care of that.

Right now i have 3-4rpm rattle. Donut gasket between header/cat is black. Could this cause rattle?

 

the catalyst in your carsound cat is probably loose.. welcome to the world of running carsound cats in untuned ITRs.. running very rich burns them out..

there's a lot of info available on this if you search..

 

my car is tuned.. and i cat seems fine when you look through. but... could bad donut gasket cause the rattle?

 

The rattle you mention is most likely the catalyst. Those things are not the best-built converters.

 

thanks, the rattle doesn't start until the car warms up, cat has 40K on it.. - when tapping/shaking cat around, nothing feels loose..

can anyone answer my other question. - can bad donut gasket cause rattle?

 

I had the same problem, so I just took my whole b=pipe/cat off and had a 2.5 inch header back b-pipe made. Either the rattle went away, or I can't hear it over my bumblebee on crack annoying exhaust tone.

 

tighten the bolts..

 

My carsound rattled just like u described. Later found out that it gutted itself

 

if you're still running the stock exhaust then check the heatshielding welded to the piping. I had to use a clamp around it.

 

usdm cat is not 2.5 inches so it will cause the heatshield to rattle.