ai86301

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?

rrwirsi

Just a guess, maybe the alternator is the culprit. To find out, disconnect the belt and see if the sound is gone.

Napoleon_Tanerite

Could be any of the pulleys in the belt drive system. Get a squirt gun and shoot each one individually with water and see if the sound momentarily changes or stops.

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ai86301

Thanks for the reply. I may indeed have an alternator issue as occasionally I'll get a "charging system fault" code which will last for a second or two and go off. No actual problem as alternator is charging and voltage is correct. I was thinking of trying the old garden hose trick where you use the hose as a stethoscope by placing one end of the hose on the suspected bad part and listen at the other end. I'll have to wait until the problem gets worse (which I'm sure it will) as it doesn't last long enough now to do any checking.
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

rrwirsi

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

ai86301

I agree with the risk of a rebuilt alternator. I'm not sure anyone rebuilds those things around here anymore. Because of the nature of the problem it might be wise to go with a new one rather than try to rebuild if they are still available. Have to wait until problem is more consistent and do my hose test or take off the belt.

Commanderr1

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.

C5MSG2004Vert

You should post a video of the hum. Might help in guessing what it is.

ai86301

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.

Commanderr1

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.

jim993

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

ai86301

Thanks again for your reply. Yeah I think I can handle that! You would use the same procedure to "remount" the belt I would assume.

ai86301

I wonder what would happen if this air pump suddenly became disconnected. I'm also in Prescott. Went to the show today. Lots O' fun.

jim993

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/

ai86301

I haven't had any codes appear with this problem. Everything appears normal with the gauges reading normal. Side note: I'm really impressed with how knowledgeable you and the other respondents are with regards the inner workings of these cars. You must have had some formal training or had a lot of trial and error experience.

ThomasR

Try removing the oil fill cap.

jim993

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.