Engine Knocking Noise
Thread Starter
5th Gear
Joined: Nov 2023
Posts: 5
Likes: 0
Engine Knocking Noise
I have a 1975 Small Block 350 V8 Corvette. I bought it about 2 months ago. It had extremely loud side pipes on it, so I don't know if the noise was there previously. I had the side pipes removed and a new exhaust built for it.
The noise doesn't start until 10 to 15 minutes after it is started. I can't tell if it speeds up with acceleration. I am hoping it is an exaust sound or other issue, but am worried it is an engine knock.
I have done the following based on threads in this forum, and elsewhere:
Replaced Mechanical Fuel Pump
Replaced Fan Clutch (it needed it)
Changed oil and filter looking for signs of metal (one seen)
Changed Fuel Pump
Checked Flex Plate (no noticeable cracks)
I used a Mechanic Stethoscope to try and pinpoint the noise, but it seems to be everywhere.
I have uploaded a video to YouTube of the sound. Any ideas are greatly appreciated. My next step is to have the engine rebuilt, but would rather not spend a few thousand on something I just bought.
Video: https://youtu.be/fk839zs_V0g?feature=shared
The noise doesn't start until 10 to 15 minutes after it is started. I can't tell if it speeds up with acceleration. I am hoping it is an exaust sound or other issue, but am worried it is an engine knock.
I have done the following based on threads in this forum, and elsewhere:
Replaced Mechanical Fuel Pump
Replaced Fan Clutch (it needed it)
Changed oil and filter looking for signs of metal (one seen)
Changed Fuel Pump
Checked Flex Plate (no noticeable cracks)
I used a Mechanic Stethoscope to try and pinpoint the noise, but it seems to be everywhere.
I have uploaded a video to YouTube of the sound. Any ideas are greatly appreciated. My next step is to have the engine rebuilt, but would rather not spend a few thousand on something I just bought.
Video: https://youtu.be/fk839zs_V0g?feature=shared
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,135
Likes: 4,245
From: US-PNW
I can definitely hear the low knock. Before the video I was pretty sure you were hearing lifter noise (constant) or over-advanced timing (on acceleration) but both are much higher pitched than what you have.
Given you mentioned a flex plate, I assume you have an automatic trans? If so, hold the brakes, put it in drive, and rev against the brakes. If it's crankshaft-related (bearings), the sound should become louder. More difficult with a manual trans, but you can lug it up an incline to get the same effect.
Could also be a loose exhaust pipe hitting the frame...
Given you mentioned a flex plate, I assume you have an automatic trans? If so, hold the brakes, put it in drive, and rev against the brakes. If it's crankshaft-related (bearings), the sound should become louder. More difficult with a manual trans, but you can lug it up an incline to get the same effect.
Could also be a loose exhaust pipe hitting the frame...
Last edited by barkingrats; Nov 6, 2023 at 10:50 PM.
Race Director
Joined: Nov 2006
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From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
Problem is. Lot's of things can cause a knock. And listening to a video is no replacement to being there.
Sorry. No help.
Take it to a trusted mechanic who has good ears and have him listen. (In person).
Sorry. No help.
Take it to a trusted mechanic who has good ears and have him listen. (In person).
Thread Starter
5th Gear
Joined: Nov 2023
Posts: 5
Likes: 0
it is surprisingly hard to find a mechanic that works on anything without a computer, that is competent. And if they are able to work on it, like a classic car or hot rod shop, they are booked out months.
Thread Starter
5th Gear
Joined: Nov 2023
Posts: 5
Likes: 0
I can definitely hear the low knock. Before the video I was pretty sure you were hearing lifter noise (constant) or over-advanced timing (on acceleration) but both are much higher pitched than what you have.
Given you mentioned a flex plate, I assume you have an automatic trans? If so, hold the brakes, put it in drive, and rev against the brakes. If it's crankshaft-related (bearings), the sound should become louder. More difficult with a manual trans, but you can lug it up an incline to get the same effect.
Could also be a loose exhaust pipe hitting the frame...
Given you mentioned a flex plate, I assume you have an automatic trans? If so, hold the brakes, put it in drive, and rev against the brakes. If it's crankshaft-related (bearings), the sound should become louder. More difficult with a manual trans, but you can lug it up an incline to get the same effect.
Could also be a loose exhaust pipe hitting the frame...
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Burning Brakes
Joined: Jan 2003
Posts: 961
Likes: 275
From: Frederick MD
I have a 1975 Small Block 350 V8 Corvette. I bought it about 2 months ago. It had extremely loud side pipes on it, so I don't know if the noise was there previously. I had the side pipes removed and a new exhaust built for it.
The noise doesn't start until 10 to 15 minutes after it is started. I can't tell if it speeds up with acceleration. I am hoping it is an exaust sound or other issue, but am worried it is an engine knock.
I have done the following based on threads in this forum, and elsewhere:
Replaced Mechanical Fuel Pump
Replaced Fan Clutch (it needed it)
Changed oil and filter looking for signs of metal (one seen)
Changed Fuel Pump
Checked Flex Plate (no noticeable cracks)
I used a Mechanic Stethoscope to try and pinpoint the noise, but it seems to be everywhere.
I have uploaded a video to YouTube of the sound. Any ideas are greatly appreciated. My next step is to have the engine rebuilt, but would rather not spend a few thousand on something I just bought.
Video: https://youtu.be/fk839zs_V0g?feature=shared
The noise doesn't start until 10 to 15 minutes after it is started. I can't tell if it speeds up with acceleration. I am hoping it is an exaust sound or other issue, but am worried it is an engine knock.
I have done the following based on threads in this forum, and elsewhere:
Replaced Mechanical Fuel Pump
Replaced Fan Clutch (it needed it)
Changed oil and filter looking for signs of metal (one seen)
Changed Fuel Pump
Checked Flex Plate (no noticeable cracks)
I used a Mechanic Stethoscope to try and pinpoint the noise, but it seems to be everywhere.
I have uploaded a video to YouTube of the sound. Any ideas are greatly appreciated. My next step is to have the engine rebuilt, but would rather not spend a few thousand on something I just bought.
Video: https://youtu.be/fk839zs_V0g?feature=shared
Get the car high up in the air and listen underneath while it’s running. A mechanics stethoscope should help you isolate it. If it’s coming from the pan then it’s the motor. If it’s coming from the back it’s the transmission.
An additional thought sprung to mind. Have you ruled out the accessory drive? If not run the car and get it hot then pop the belts off super quick and see if the noise changes.
Drifting
Joined: Oct 2020
Posts: 1,567
Likes: 576
From: Wisconsin
Timing chain slapping inside the cover from being too loose? Those years had the nylon top gear that wore out and the chain would become very loose (but still run ok).
I’m only hearing it on the decel and idle. A lower end knock would be more constant, I would think.
I’m only hearing it on the decel and idle. A lower end knock would be more constant, I would think.
Pro
Joined: Nov 2014
Posts: 728
Likes: 346
From: Smyrna, TN
This is why these knocks can go away with motor honey or STP...(Gear oil)
Timing chain sound is like taking a chain and hitting sheet metal... tinny.
Lifters have a higher pitch "clack" sound.
Rods and Main bearings sound deeper, like a hammer hitting the block
Team Owner
Joined: Mar 2002
Posts: 44,675
Likes: 1,830
From: Huntsville AL
I have listened to your video several times and it's not characteristic of a rod knocking but more in line with valve train noise, that said my head is not under the hood so this is just a guess on my part. For a rod knocking (failed bearing) I would expect to hear that low thunking sound at idle but also hear it with the rpm increased, just a faster thunking sound.
I would pull the valve cover and have a look there before dropping the oil pan. With the knocking sound, is it more pronounced from under the car or on top of the engine?
I would pull the valve cover and have a look there before dropping the oil pan. With the knocking sound, is it more pronounced from under the car or on top of the engine?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
First order of business with any unusual engine noises is to check what you CAN see then worry about what you can't see later on.
As Mr. D stated, pull the V.C.
You can buy oil retaining clips that fit on the rocker arms. Those will keep oil from spraying on your exhaust system. Then, using a strip of thick cardboard, insert that between the valve springs and the valve cover bottom ledge. Start it up and observe.
The oil clips are sold in packs of 8. Check one bank of cylinders at a time and do not rev it up.
As Mr. D stated, pull the V.C.
You can buy oil retaining clips that fit on the rocker arms. Those will keep oil from spraying on your exhaust system. Then, using a strip of thick cardboard, insert that between the valve springs and the valve cover bottom ledge. Start it up and observe.
The oil clips are sold in packs of 8. Check one bank of cylinders at a time and do not rev it up.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
I have an old for 5.0L engine that made that noise at idle that otherwise ran well. I changed to 0w-40 M1 oil and the noise went away. I suspect worn main bearings with clearances that have opened up. Does the noise resolve or get better at higher RPM's? A rod knock will get louder or worse under a load.
Try the simplest solutions first. Change the oil wt. You could go 15w-50 and see what happens. Ultimately the mains are probably worn, but they can last a long time that way if the engine is not subjected to hard use.
Try the simplest solutions first. Change the oil wt. You could go 15w-50 and see what happens. Ultimately the mains are probably worn, but they can last a long time that way if the engine is not subjected to hard use.
