Hello guys,

Recently I've started to notice this werid rattle under the car. The sound dosn't seem at all audible from the front (the engine bay). Seems to get louder as you progress to the back of the car. It's also mostly inaudible after certain RPM (~3,000 RPM or more). Another thing I've noticed is that it dosn't matter if the car is hot or cold, it will very intermittedly happen while idle. I can't seem to pin point where this could be coming from. I've attached videos, one by the middle of the car (pointing towards the cat) and one to the back near the exhaust. Any ideas as to what it might be?

Thanks,

 

Guessing exhaust. Most likely broken cat internals.

Get under there, engine off, and use your fist to bang exhaust front to back, trying to reproduce the sound.

Once you locate where its coming from, should be easy to pinpoint cause.

 

I also suspected the cat could be the issue, I have hit it a couple times with a rubber mallet, but I can't consistently gotten it to rattle, sometimes I can hear it faintly if I hit it towards the front of the cat. Wouldn't a broken cat have a more pronounced sound if it's hit? Granted... I didn't take of the heatshield from the cat.

 

If it’s the cat, hitting the front flange with a good sized rubber mallet will make it rattle with every hit.

 

I'll try this tonight, and thanks for the recommendation.

Do cats fail for any particular reason? My car is an 04 but it only has around 53,000 kms (~33,000 miles) on the odo.

 

If its a stock cat, usually only fails with broken media if there is a heavy mechanical impact.

 

So... I actually checked the car and hit the flange a couple times, it seems like when hit there it can be faintly reproduced, it's not quite as audible as when the car is on.

What is the recommendation for the cat? Go OEM? Should I also replace the O2 sensor?

Thanks!

 

Even a used oem cat is like $500 these days. An aftermarket HFC, High Flow Cat, is much cheaper. No real performance gains, unless you pair it with a tuning solution that allows lowering vtec rpm.

You'll be able to confirm its the cat if you remove it, and shake it.

Since its not cut and dry yet that its the cat, I'd try the mallet farther up, the header, etc. Maybe a loose heat shield, a broken weld, etc. Also check the S shaped bracket that mounts header to block. Maybe loose bolt, etc.

Also check battery heat shield under hood.

 

Could ask Billman,he has a secret way to make silent via the sensor ??
I used a screw<3> method drilled into side of media/s,could squirt fire gum via drilled holes also..
Prices have gone through the roof as being stolen for scrap values.....

 

Good point, nothing to lose trying to fix the one you have. If it doesn't work, nothing lost, and cat still has its full scrap value.

The various fixes involve immobilizing the inner media, so ot can't rattle.

When the media breaks free from outer walls of cat, and rattles, its typically entirely still functional. The media chemically interacts with exhaust gases, and the cat material merely needs to be present for that to still take place, regardless if its bouncing around and rattling.

So long as cat doesn't become clogged or media contorts itself so as to impede exhaust flow, its all good (except for the embarrassing noise).

So yeah,if its cat media, try one of the media mobilizing tricks.