I have had a new exhaust for a while now, with a stock Cat. Just recently, Whenever I accelerate, a terrible rattling eminates form my undercarriage, I thought it may have been the hangers, but they are all secure. It sounds like it's coming from my cat. Can I just replace the cat w/ a test pipe? What effect would this have?

 

Better check with your state emmisions people to see if you get a large fine $ 500. or better in your state for not having one on your car...

WheelBrokerAng

 

Did the noise start at the same time you got your new exhaust or while after? It is possible that the filter element in the cat has come loose and is rattling around in there. I would make sure first. Check all the clamps and bolts to make sure they are tight. Also make sure that all your heatsheilds aren't bent. Look for shiny spots on the heatsheild and on you exhaust to make sure they are not rubbing somewhere. When you hit the gas does it rattle? Do you have to be driving? If you can get it to rattle while in park, see if you can have a friend listen, just be careful.

 

If the noise started right after you installed your new exhaust, it's probably the heatsheild. Mine started vibrating off and on right after I installed an Apexi WS 2 catback on my 96 Accord EX E22. First, jack the car up when your exhaust is cold, then get underneath and grab the exhaust pipe after the cat and shake it back and forth. If you hear any contact noise then it's probably your heatsheild. The metal of the heatsheild is soft so just locate where the contact is being made and use a 1x2 piece of wood cut to maybe a foot long to wedge between the exhaust pipe and the heatsheild to bend the heatsheild out of the way. Tap on the end of the wood with a small hammer if you need to. Shake your exhaust back and forth again and keep repeating this until you have complete clearance with no contact. Also make sure none of the hanger hooks are making the contact with your exhaust either. I had a problem with the one in back of the resonator right before the first 90 degree bend. I found a rubber endcap to put over the end of the hanger hook that was making the contact noise and this solved the problem. The hanger hook still touches the exhaust but the rubber endcap got rid of all the noise and the rubber never melted either. My exhaust now has no vibration or noise at any rpm and the noise was pretty bad when I first installed me Apexi WS 2 exhaust.