Strange "Knocking" Sound
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From: Wilmington NC
Strange "Knocking" Sound
I drove the vette to work this morning and while sitting at a stop light I noticed a "knocking" sound. When I accelerated to pull away, it went away. When I got to the office, I noticed that I heard it as as soon as I opened the door to get out of the car. I raised the hood and couldn't hear anything coming from the engine bay. I walked back by the drivers door and bent down and it sounds as if it is coming from under the car just in front of the drivers door.
Any experience anything like this before?
Any experience anything like this before?
Le Mans Master
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From: indy indiana
check you oil pressure at idle,does it drop a bunch? like way to much? like almost zero? if thats the case,its rod bearing(s) knocking on your wallet.Hope its something else,something cheap.Put it on a lift at idle,find out where its comming from. Also need more info,heck,it could be a exhaust pipe hitting the frame.
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From: Wilmington NC
Originally Posted by ghoastrider1
check you oil pressure at idle,does it drop a bunch? like way to much? like almost zero? if thats the case,its rod bearing(s) knocking on your wallet.Hope its something else,something cheap.Put it on a lift at idle,find out where its comming from. Also need more info,heck,it could be a exhaust pipe hitting the frame.
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From: Wilmington NC
OK, I went out to the parking lot and started the car. I drove it up the road about 3 miles and back. When I parked it I could hear the noise. My oil pressure was fine while driving and was hovering bwteeen 10 & 20 lbs while idling. I slowly pushed the accelerator down and the speed of the sound would increase but not to the same speed as the engine RPM's if that makes since.
Le Mans Master
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From: Louisiana
On a 85 vette I had I could hear a knocking sound that didn't match the RPM of the engine. Mainly I could hear it at idle and when accelerating I didn't hear it. Seems I could hear it on the passenger side more. It turned out to be the flywheel. Just an experience post. Hard to say if we cannot hear it.
Melting Slicks
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On my '85 I had a knocking sound when I got the car.
I would hear it at idle, only after the car was warm, then it would disappear when I was moving. It sounded like it was in time with RPM's. I got under the car while it was running to pinpoint the noise- turned out to be coming from the (only) cat. Fun part was, the cat also puked into my mufflers.
I would hear it at idle, only after the car was warm, then it would disappear when I was moving. It sounded like it was in time with RPM's. I got under the car while it was running to pinpoint the noise- turned out to be coming from the (only) cat. Fun part was, the cat also puked into my mufflers.
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From: Wilmington NC
Originally Posted by mikey whipreck
On my '85 I had a knocking sound when I got the car.
I would hear it at idle, only after the car was warm, then it would disappear when I was moving. It sounded like it was in time with RPM's. I got under the car while it was running to pinpoint the noise- turned out to be coming from the (only) cat. Fun part was, the cat also puked into my mufflers.
I would hear it at idle, only after the car was warm, then it would disappear when I was moving. It sounded like it was in time with RPM's. I got under the car while it was running to pinpoint the noise- turned out to be coming from the (only) cat. Fun part was, the cat also puked into my mufflers.
Le Mans Master
Joined: May 2006
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From: indy indiana
hopefully,its something simple and cheap..like exhaust system knocking against something. flywheel/converter noises make a knock as well but ususally increase as rpm increases,but not always. best of luck.
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From: Vandenberg AFB California
Same deal
I just posted a thread about this in the Tech section.....I have the same knocking, so I put it on a lift and sure enough, both the drivers side pre-cat and the main cat are ticking/knocking loud. If you get under it, you will be able to tell if that is it.
Le Mans Master
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From: Springfield PA
Philly 'Burbs Events Coordinator
If you have a 6sp ZF trans, see if it goes away when you put in the clutch. ZFs are noisy! I heard the same exact thing you described an hour after I bought my Ruby 7 years ago. I paniced because I thought it was a rod knock. 65K miles later and never a problem 
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From: Wilmington NC
Stopped by the stealership on the way home. I guess I should be nicer, the corvette mechanic there is actually very good and great to work with. Anyway, he stopped working on the car he had in the bay and came out to give it a listen. He even took the car out and we drove it around for about 30 minutes. He said that if he had to guess he would put his money on a cracked flywheel but wouldn't rule out a lower end problem. He said that there was a slight chance that it could be the cats but it didn't have the "right" sound to him. He said he was going to put it up on the rack Tuesday and look at the flywheel first. If that was fine he'd drop the oil pan and get in and look at the bearings. He said that if it was his car and the bearings were shot he'd never spend the money to fix them, he just buy a short block.
(edit: I forgot that at the end of our drive he had me "power brake" the car so he could hear it under load. As the RPM's increased to about 3,000 RPM's the "knocking" stopped. I then brought it back up to about 3,000 RPM's under load and then SLOWLY backed off the trottle. The knocking would start back at 1500 RPM's every time. He sai dthat if it was the lower end it should knok even at higher RPM's and thus why he thinks its the flywheel.)
(edit: I forgot that at the end of our drive he had me "power brake" the car so he could hear it under load. As the RPM's increased to about 3,000 RPM's the "knocking" stopped. I then brought it back up to about 3,000 RPM's under load and then SLOWLY backed off the trottle. The knocking would start back at 1500 RPM's every time. He sai dthat if it was the lower end it should knok even at higher RPM's and thus why he thinks its the flywheel.)
Last edited by beachrover; Sep 22, 2006 at 08:47 PM.
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From: Wilmington NC
More Issues
Originally Posted by beachrover
Follow up: Mechanic just called and it was the main CAT. Flywheel, bearings all looked fine. He said that when he unbolted the exhaust rust spilled all over the floor.
Any ideas?
Drifting
Joined: Oct 2004
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From: The Woodlands TX TX
Originally Posted by beachrover
I guess all things being equal, almost a grand.
Nice avatar! She someone special?
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From: Wilmington NC
Originally Posted by USAFPILOT
Now is a good time for some high flow cats, maybe some long tube headers, if your spending that kind of money get some better parts than original.
Nice avatar! She someone special?
Nice avatar! She someone special?
Tera Patrick
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From: No Opti Here Tn
St. Jude Donor '06
Im thinking if you reboot the computer it will be fine
It is used to the exhaust restriction and now it is gone
You could need O2 sensor(s) also
There is a way to do it but disconnecting the battery for a few minutes will do it too
It is used to the exhaust restriction and now it is gone
You could need O2 sensor(s) also
There is a way to do it but disconnecting the battery for a few minutes will do it too
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From: Wilmington NC
Took the car back to dealership. They called this afternoon and wouldn't you know it, they say it starts and runs fine. They said there are no codes stored or anything else they can find wrong. They asked me if possibly I had "bad gas". I told them the car was starting fine before the changed out the CATS and I haven't put any gas in it since then. I told them to keep it over night and try it again in the morning.
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From: Wilmington NC
"Service Advisor" just called. They did have difficulty starting the car this morning. They checked the fuel pump, injectors, vaccum lines, gaskets no problems could be found. Reset ECM, drained gas tank and refilled tank, no problems. Finally an old retired mechanic comes in and ask them if they've checked the trottle body. They pull it off and bingo, "dirt" and various other build up. Said they will clean it up & reinstall in the morning.
Sound like this could be the problem?
Sound like this could be the problem?