Help needed! Noise coming from under car.

[Jonnydext]

Hi all,
I'm new to the form.
I just bought a 96ce lt4. It has very low miles 20k looks and drives great . But after a few hours a wierd knock noise started coming from the back of the engine, near the clutch and cat area.
I was able to get it on a friends lift and take a video it seem like it's the cat?
Any help is much appreciated as I just bought the car
c4 corvette knocking noise - YouTube

 

[Hib Halverson]

It's often tough to diagnose from video footage, but while it is possible the driver side cat has a problem, I kind of think that noise is coming from inside the engine.

You can get a much better fix on from where the noise is coming with a good mechanic's stethoscope like this one.

If that rattle is coming from the left-side cat, obviously, you have to replace the cat because the substrate inside it has fractured.

 

[Jonnydext]

I had originally thought It was the engine ... but the sound doesn't change or increase with engine speed ,just vibrates more. It also goes away when I break hard or park on a steep nose down incline at idle. ( will come back if I rev)
it's gotten worse sense the video ... it really sounds like heat shield vibration... but all we're tight when I checked .. I've heard a problem cat can effect gas mileage.. I've been getting 9 mpg , but it's new and I've driving it like it should lol

 

[Jonnydext]

And thanks for all the insite !!

 

[Hib Halverson]

If the noise doesn't change with engine speed, then, yeah...it's unlikely an engine mechanical issue.

Might as well, pull the exhaust and the cat on that side and inspect it.

 

[Wayback]

Wayback in the day we would use a old broom handle,put 1 end on the part etc & the other end in the palm of your hand & ear,you'll be surprised how much you'll hear that way if ya dont have access to a stethoscope.

 

[kpic]

Wayback in the day we would use a old broom handle,put 1 end on the part etc & the other end in the palm of your hand & ear,you'll be surprised how much you'll hear that way if ya dont have access to a stethoscope.

Way back in the day, I learned the value of a piece of aquarium hose with M/C multi-carb setups.

 

[Hib Halverson]

"Way back in the day", I tried both of those methods, too, and, yeah...they work better than using nothing but your ears, but today, that Lisle automotive stethoscope kit is what I use. It works far better than either the two methods discussed above.

Anyone making a serious effort to track down a noise source which is difficult to locate or is trying to hear a noise which is subdued needs a tool kit like that.

 

[Tuna]

I have a stethoscope and it has definitely come in handy.
I also have a 2-foot long screwdriver that I've used in the past. I put the tip of the screwdriver wherever I think the noise is coming from then put my ear to the handle. Works pretty well.

 

[kpic]

I have a stethoscope and it has definitely come in handy.
I also have a 2-foot long screwdriver that I've used in the past. I put the tip of the screwdriver wherever I think the noise is coming from then put my ear to the handle. Works pretty well.

The screwdriver has worked well for me also..
I learned the aquarium hose trick from a mechanic who tuned a lot foreign multi-carburetor setups.. It converts the omni directional ear to directional..

Old school works right well so does the new ways. Knowing what the noise means is the secret to either..

 

[Jonnydext]

I ended up taking the cat off , it was nfg. The passenger side was starting to make a little noise too.
here is the link to video
YouTube

While I was under the car I noticed a reman tag on the trans,is this something I should be concerned with?

 

[Jonnydext]

Sorry the first link didn't post
96 c4 broken cat - YouTube

 

[Tuna]

The cats on my old '92 started coming apart many years ago. I noticed a rattling noise when I was at an ATM or fast food drive through with the window down.
Sounded like a coffee can (tin can) full of marbles being shook.
Glad you figured your noise out.

 

[Hib Halverson]

My guess is you're going to end-up replacing both cats. It's alarming that both cats need to be replaced with only 20,000 miles on them, but s**t happens.

As for the remain tag on your S6-40...that's a good thing as it means the trans was overhauled and has less mileage on it than does the engine.

 

[Jonnydext]

The cats on my old '92 started coming apart many years ago. I noticed a rattling noise when I was at an ATM or fast food drive through with the window down.
Sounded like a coffee can (tin can) full of marbles being shook.
Glad you figured your noise out.

Thanks... it sounded just like that.
I'm going to replace both ... at least I posted some videos now, so some else can maybe put there minds at east quicker then I was able to!

 

[Jonnydext]

My guess is you're going to end-up replacing both cats. It's alarming that both cats need to be replaced with only 20,000 miles on them, but s**t happens.

As for the remain tag on your S6-40...that's a good thing as it means the trans was overhauled and has less mileage on it than does the engine.

Thanks Hib
Would they most likely have just rebuilt the original one and put it back in ...
Are matching numbers even a legitimate concern in this genre of Corvette ?
Just curious