Clattering (exhaust?) noise in front

[mc2718]

Can a rattling/clattering noise coming from the front bottom of the car
(ahead of the driver, down below) be caused by exhaust sections BEHIND
the driver (cat converter or even muffler)?

I had exhaust work done on a '91 Corolla and got the flex pipe and
muffler replaced (both were rusted away). After this, the car did not
sound like an airplane any longer (which was the good part), but some
strange rattling noise is coming from the front, somewhere down below,
when I accelerate. Sometimes it is very loud, sometimes it only appears
when I accelerate really hard - cannot figure out what it depends on
(perhaps weather?). I took it back because I thought they botched the
flex pipe installation but the mechanics said it must be the cat
converter heat shield (the top half was missing already). So they got
rid of the remaining bottom part but that made negligible difference.
Then they took a longer look, did some tests, and told me that it was
the inner pipe in the cat converter, which was not attached properly by
whoever installed it (this is an old car, the converter was welded onto
the pipes years ago). They said they drilled some holes and redid the
welding to secure that inner pipe, and the noise indeed went away.
But only for a week or two - now it is coming back again. Could this really
be that inner pipe of the cat converter? Because based on that the noise is
coming from the front, naively I would suspect that flex pipe...

[90 GP]

From your description it could be the engine pinging due to a buildup of carbon or improper timing. Try a higher octane grade of gas and see if it quiets down.

[SCT]

It could be the same problem again if what they did solve the problem the first time. Rust could be killing the weld they put in since the Cat produces water.

Part of the exhaust you may want to check is where the exhaust goes over the center crossframe under the motor. I had that problem. Some exhaust hangars may not be welded in the correct exact places which allows the exhaust piping to sit higher or lower than it's suppose to. When you accelerate, the motor tilts just enough to move part of the exhaust system to contact something.

[mc2718]

Many thanks for the quick replies, I will try out the suggestions. By the way, could this problem be "harmonic resonance"?

[SuperRA]

Sounds like the exhaust heat shields to me. I've had this happen to several cars. There are 4 or more shields on the AE92, you'll have to check each one to make sure they are tight. Just the slightest movement can make a noise and it's only at certain RPMs like you mentioned. The loose shield acts like a tuning fork and at the right frequency (or wrong freq?) it hits its resonant freq and gets real loud. Sometimes the bolts rust away, I've used stainless steel tie wraps or copper electrical wire to fix this temporarily.

[mc2718]

Now the most typical sound is like this http://sites.google.com/site/mc2718/...edirects=0&d=1 (ignore the turning signal clicking at the beginning). It comes up when I accelerate, unless I do it *very* slowly. There is also somewhat similar sound but without rattling when I let up the accelerator pedal (i.e., the car is decelerated by the engine break).

Is there heat shield at any other place than the catalytic converter? The cat converter's heat shield has been completely removed...

[Rookie_One]

there are 4 heat shields on the entire exhaust line in fact....

[mc2718]

I took it back, and they fixed it - hope it lasts. Probably a slight miscommunication, it was/is a problem with those double-wall pipes. The inner wall was loose and rattled/resonated. They did some welding to keep it in place. If it started rattling again, they told me they would replace the pipes.

[alfaffa]

Originally Posted by mc2718

I took it back, and they fixed it - hope it lasts. Probably a slight miscommunication, it was/is a problem with those double-wall pipes. The inner wall was loose and rattled/resonated. They did some welding to keep it in place. If it started rattling again, they told me they would replace the pipes.

Was it the flex pipe or the muffler that they welded to get rid of the rattling?

[mc2718]

Neither. It was the pipe section near the cat converter - cannot remember whether "before" or "after" it though.

[mc2718]

It did last only 500 miles or so only, though. Will have to get the pipe replaced eventually, I think.