Hi everyone,

I am a newbie on this forum. My 99 Accord EX 4 cyl. with 110K has developed a rattle, or tapping noise. It sounds like it is
cooming from the catalytic converter. When the car is idling (in gear or park) it makes a fairly loud rattle. If you increase the rpm's
to 1000 it stops.

At first the noise scared me to death. It sounded almost like a lower engine tapping. However the car runs great, no check engine
lights, no apparent blockage in the converter.

By the way the car has a ULEV (ultra low emissions) sticker in the rear side window. I don't know if this means it was a Cali car
originally or not. However, I did read a post that said a standard or non- Cali converter would fit with no modifications. Hopefully
this is correct.

Thanks in advance for your help

 

A heat shield might be loose on the exhaust somewhere, there is one on the cat. I would start by checking there.

 

exactly what I was thinking. This happened with my civic

 

Thanks for the responses. It's definitely not the heat shield. That was my first impression also. However, I've been under the car, and there is no play or vibration in the heat shield. I even banged it a little with a small hammer, and it still didn't move.

The sound is more like an internal noise. It's not really like a typical catalytic noise, but it doesn't sound like a typical engine noise either. It doesn't affect the running, or performance, but it's definitely getting louder, and is a nuisance.

Plus my wife is the primary driver - so you can figure out for yourselves how much I'm hearing about this.
Seriously though. It is driving me crazy while I'm waiting for something severe to happen.

 

If you take apart a Catalytic converter you'll notice a bunch of "honeycomb" looking stuff inside it. On RARE occassion if a piece of this brakes off or comes loose it can cause that rattaling noise.... I would be more prone to think you have a small exhaust leak that's blowing on a thin piece of metal and it would sound funny.... I doubt/hope this isn't it but it's a thought.... you'd have to pull off your cat and shake it to hear this most likely...

 

Thanks Sir nasty,

I'm familiar with the honeycomb inside. Aren't they usually ceramic or some non-metal material. The puzzling thing is the metallic sounding rattle/tapping noise, and the fact that it stops at 1000 rpm (in gear or not). At this point I'd be happy to replace a converter, I just don't want to find out it is a major internal engine issue.

Any idea how much internal engine pressure there would be in a 99 4 cyl? If you remove the oil filler cap while the engine is running
there is a fair amount of internal air pressure at the opening. I'm assuming this is typical, although my miata 4 cyl does not exhibit any pressure under the same conditions.

 

the reason why it only rattles at a certain rpm is because that is the resonate frequency of that particular object. just like how a wine glass breaks at only a very high pitch. thats the resonate frequency of the glass. rpms/sound/pressure frequencies above or beloiw that certain area just dont rattle.

my first guess would have been the cat shield but if you are sure that it is not the source of the rattle, then it is probably the cat itself and it should be replaced. any universal catalytic converter will work and will pass emmissions for you.

 

Good call lightshow.... I didn't even consider that... I would be willing to bet it's your cat as well then... just pull it off and shake it around, if it makes noise well then you know that's the issue and it'll be easy to replace since it's off *L* If you can afford it go and buy one, pull yours off and shake it around, if that's the problem replace it, if not take back the new cat...

 

What was the outcome of this problem? Did you end up replacing the converter, or did you find the rattle? I'm experiencing the same problem at the moment.

 

There's lots of thin heat-shields around the exhaust system. Have you checked & ruled these out?