mysterious "tone" or hum while engine is running
Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
mysterious "tone" or hum while engine is running
My vette has 59,000 miles on it. I have an intermittent problem with a "tone" or 'hum' when the engine is first started which lasts from 2 to 5 minutes then stops by itself. It's fairly loud. It seems to disappear if you rev the engine and will reappear when you back off. It does not change pitch when you rev. All the gauges read OK and the engine runs fine when the tone is present. I get no "error" messages from the computer. The tone is not a high pitched squeal as from a glazed belt. The pitch is about an "A" below middle "C". It is not a valve train or clatter type noise. It seems to fill the engine compartment making it difficult to isolate. It is not a fan running. It's almost as though you ran some type of audio generator completely independent of the engine, next to the car and turned it off after 2 minutes. Any ideas?
Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
Would you have an idea of what in the alternator would cause it to make this noise? I would think bad bearings would be a constant noise and not go away, maybe also increase in pitch as the engine is revved.
Carl
Melting Slicks
Joined: Nov 2014
Posts: 3,399
Likes: 1,339
From: Havre de Grace Maryland
Carl, not a electrical engineer so your guess is as good as mine. Maybe one of the other members can chime in on your thought. But my SWAG, the hum you describe would be something electrical. If it was mechanical I would think it would squeak, not hum. But, what do I know. If it is the alternator, prior post have warned not to replace with an aftermarket one. If possible, have it rebuilt. Search the forum on this issue. Hope you find the cause and will not break the bank to fix.
Ron
Ron
Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
Carl, not a electrical engineer so your guess is as good as mine. Maybe one of the other members can chime in on your thought. But my SWAG, the hum you describe would be something electrical. If it was mechanical I would think it would squeak, not hum. But, what do I know. If it is the alternator, prior post have warned not to replace with an aftermarket one. If possible, have it rebuilt. Search the forum on this issue. Hope you find the cause and will not break the bank to fix.
Ron
Ron
Advanced
Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
I would take the belt off and start turning each pulley. Put a little pressure on each pulley as you turn it back and forth paying attention to how it feels. Should be very smooth feeling. If any of them have a rough feel or a grinding feel....even slightly it has a bad bearing and could cause the noise you are hearing.
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Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
I would take the belt off and start turning each pulley. Put a little pressure on each pulley as you turn it back and forth paying attention to how it feels. Should be very smooth feeling. If any of them have a rough feel or a grinding feel....even slightly it has a bad bearing and could cause the noise you are hearing.
Advanced
Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
Thank you for your reply. I also noticed that it will not produce the noise after sitting all night. It will only do it after the car has been started, thoroughly warmed up, stopped, been sitting for awhile and then started again. I think your suggestion is valid and I will try it when the problem gets worse. It's somewhat inconsistent now and will be easier to find if it will do it regularly. Today it behaved itself and never did it at all. I've never had the belt off so will have to check out how to do that. Re-tensioning the belt could also be interesting as I've never done that before either.
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
Another possibility is the auxiliary air pump. It runs for a short time after start, pumping air into the exhaust manifolds to reduce startup emission. On the driver side low near but under headlight. May be going out.
https://duckduckgo.com/?q=c5+corvett...oWvA&ia=videos
https://duckduckgo.com/?q=c5+corvett...oWvA&ia=videos
Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
The belt is equiped with an automatic tensioner so there's no guesswork there. The tensioner is located on the passenger side...has a 15mm bolt holding the pulley on. Put a wrench on the bolt straight up vertical and push it towards the driver side. That will release the tension...hold it there while you remove the belt being careful not to get your fingers stuck under the belt while holding the tensioner. Pretty simple.
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
If the secondary air pump is not working, you will get 1415 and 1416 codes. They will reset and not reappear if the pump and check valves are functional again.
https://www.motortrend.com/how-to/16...a-c5-corvette/
https://www.motortrend.com/how-to/16...a-c5-corvette/
Thread Starter
Cruising
Joined: Jul 2022
Posts: 10
Likes: 0
From: AZ
If the secondary air pump is not working, you will get 1415 and 1416 codes. They will reset and not reappear if the pump and check valves are functional again.
https://www.motortrend.com/how-to/1602-how-to-fiex-secondary-air-injection-code-on-a-c5-corvette/
https://www.motortrend.com/how-to/1602-how-to-fiex-secondary-air-injection-code-on-a-c5-corvette/
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
What I am saying is that if you disconnect the secondary air pump to see if it is the origin of the hum you hear at startup, no long term damage will occur but you will get the 1415, 1416 codes (and a check engine light). If the pump is the cause, I would reconnect it and get a new one to put in at your convenience. C 5 smog checks in most states will not pass if the secondary air pump or system check valves are not operating, Some tune out the secondary air system and remove it, but again, the car will not pass smog in many states in that condition.