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Less than two miles into my morning commute today, I suddenly heard a loud rumbling sound that seemed to come from either the back of the engine, or beneath the car (located roughly under the dash). It basically sounds like a juiced-up hot rod right now, although the exhaust and temperature guage look normal. When I put my foot on the gas, the rumbling gets louder with the revving of the engine.

I had a new cat welded on 18 months ago. I asked the muffler shop to put on the cheaper (non-high flow) cat converter, which only cost $100 less than the high-flow. Soon after I got the replacement, it has accelerated sluggishly and made a rattling noise every time I started the car. The shop owner said this one would be good for about 8 months, so I know I've been on borrowed time.

Any advice? I'm thinking it's the cat making the noise and should be replaced, but could it possibly be something else?

A rattling noise from the cat is a pretty common symptom of it going bad. While it's possible there could be other causes, like the down pipe, from your description, it sounds more like the cat. Put the car on ramps, or jack stands and give the cat a kick...see if that's where the noise is.

sounds like a exhaust leak. i would look under the hood with the car on and under the car. if it sounds a mean muscly then i would think the exhaust is getting out somewhere. the rattle could be a bad cat or a heat shield. should be easy to tell once you can look under the car with it on a ramp or jackstands.

S

Thanks for the advice. I ended up taking it back to the muffler shop that put the cat on. He put a short hose between his ear and the cat (while the car was running and raised on a lift)---(why didn't I think of that?). He figured out that there was a cap or plug on the top side of the cat that had unscrewed over time and then fell off in the middle of the road. That was when the clanging noise stopped and the loud engine noise started up. He screwed on a new cap in its place and the extra noise stopped immediately. Pretty shoddy workmanship, I think. But at least he didn't charge to fix it. He welded it into place this time, so hopefully it won't fall off again.

It turns out that the cat was actually a "higher" brand catalytic converter. He told me it should be good for a couple more years.

I had a magnaflow put on my car 2 years ago, and it only ran me $70 or so for the cat, along with some other exhaust work I had done. Granted, I did have the PCM reprogrammed to not throw a code, and I don't have a rear O2 sensor. I also don't have any emissions testing to deal with.