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Vehicle: 1997 Jeep Grand Cherokee (2WD) <-- :(

Issue: Makes a sort of 'grinding' sound when it's running. It appears to be RPM dependent, and seems to be the worst at low RPMs (idling is the worst). The noise is a sort of grating/grinding. It sounds a little "echoey", so to speak, like something rattling against a hollow chamber. When I start the car or shut it off it is the loudest and scariest sounding, then it dies down a bit. Noise comes from under the car (maybe toward the back? It's hard to tell really over the engine noise).

Ideas: As far as I've researched so far, my best bet is a loose heat shield on the catalytic converter. (I suppose this would explain the rpm dependency if it only makes noise at certain engine vibration frequencies.)

Can anyone confirm this, or offer another explanation? It seems to have worsened as of late. The noise just appeared one day when I started it, although I suppose it's possible it was going on earlier just not loud enough to be noticeable.

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You're probably right, but it could also come from the inside of any of the exhaust components. A failed catalytic converter can be VERY noisy. Once I had a failing front-muffler/resonator too and it was producing a rattling sound at low speed.

That would also explain the echoey sound.

The best way to determine that is to hit every single exhaust component with your fist or a rubber hammer and listen for any rattling sound, but since it's RPM dependent, you might not hear anything by doing this with the engine off.

If you have access to car stands or a lift, lift the car, ask somebody to accelerate a bit and inspect the exhaust while the engine is running and you can actually reproduce the noise.

Just be careful you don't touch the exhaust with your bare hands as it can get very hot.

Good luck!

NB: if you've never changed your catalytic converter in almost 20 years, there's a good chance it's cooked and hitting it will immediately reproduce the sound as metal chunks would be completely loose inside.

  • Great I'll try messing with it as soon as I'm home from work, don't exactly feel like spending my lunch in the parking lot banging on my car! If it turns out to be the catalytic, is that difficult to replace? I've done the starter and a few other parts on it, so I've got a but of experience but I wouldn't call myself experienced, so to speak. – thekeegs Jul 2 '15 at 17:08
  • Replacing a catalytic converter is a fairly simple job. If the exhaust is stock, it's usually bolted between the headers and the catback. Some have O2 sensors screwed in so you might need a special O2 sensor socket to remove them, but a spanner would do the job too. Maybe you'll need a second person to help you while the cats are off as the catback might not be retained by anything else, but an old metal coat hanger might also be a good temporary exhaust hanger ;-) – Capsule Jul 2 '15 at 23:09

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