Whistling only at idle driving me nuts...help?

[derekjl]

New to the forum here. I recently picked up my forth OBS Tahoe. Had three '99s and now a '95 with the TBI. Definitely a different beast but it had 66k miles on it, one owner, and a price I couldn't refuse. Now, something has been bothering me since I picked it up. There is a distinct high-pitched "whistle", almost like a tea kettle boiling, coming from the engine. This only occurs at idle and when coming to a stop from 1,000rpm down to idle. The pitch changes with the engine rpm, but again only below 1,000. If it is above that, there is no noise at all. At first I thought it was the alternator, coincidentally the alternator did go bad and I replaced it with an AC Delco unit, basically the same one you'd get at the dealership. The noise was still there, but now it was even louder. I cannot say 100% it is definitely coming from the alternator, but it is in that general area of the engine bay. Something key here, the more electrical items that get turned on, the louder the noise is. So, if I have the fan going, the high beams on, and the rear defrost on, it is really easily noticeable. But, if nothing at all is on and just the engine is running, it is almost not there. I'm really stumped on this one. I'm a novice at best when it comes to this kind of thing, it's just the noise itself is extremely annoying, especially in stop and go traffic. Any input is appreciated. I made a video to show the exact noise and general location...

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[delta_p]

my '96 C1500 does that and it's the alternator. Been through about 3 over the years from the original, an autozone brand, and using a DB Electrical brand now. All did it. Sounds like a supercharger.

 

[Dropped88]

Every gmt400 I've owned has done that with the exception of the one I ls swapped

 

[derekjl]

So this is considered normal? These trucks did this when they were brand new? I posted the same thing in another forum and it was suggested to me to check all of the ground connections. I've also heard it could be a vacuum leak and even just the injectors making noise.

 

[Oldblue98]

my '96 C1500 does that and it's the alternator. Been through about 3 over the years from the original, an autozone brand, and using a DB Electrical brand now. All did it. Sounds like a supercharger.

Just replaced one myself, kept looking in the mirror for the emergency vehicle, sounded like a distant siren at certain rpm.. But when you get old you hear **** and it sounds like it is coming from everywhere lol. New alternator made the sirens go away !

 

[c1500deathtrap]

Why don't you whistle at wide open throttle to?

 

[derekjl]

Just replaced one myself, kept looking in the mirror for the emergency vehicle, sounded like a distant siren at certain rpm.. But when you get old you hear **** and it sounds like it is coming from everywhere lol. New alternator made the sirens go away !

Mine is at idle only. The old alternator that started going bad did exactly as you said, all over the RPM range while driving. Something else worth mentioning...I notice my voltage meter fluctuates ever so slightly with my turn signal. When the signal comes on, the voltage drops, when it turns off, it goes back up. When I have my hazard lights on and the fan running at full speed, you can hear the fan slow down briefly when the hazards come on, and then return to normal speed when they go off. I'm convinced that 100 amps isn't enough for these trucks, even in stock form. So, this is still considered normal because "they all do it"?

 

[arrg]

Mine is at idle only. The old alternator that started going bad did exactly as you said, all over the RPM range while driving. Something else worth mentioning...I notice my voltage meter fluctuates ever so slightly with my turn signal. When the signal comes on, the voltage drops, when it turns off, it goes back up. When I have my hazard lights on and the fan running at full speed, you can hear the fan slow down briefly when the hazards come on, and then return to normal speed when they go off. I'm convinced that 100 amps isn't enough for these trucks, even in stock form. So, this is still considered normal because "they all do it"?

I had the same thing happening on my 88. I cleaned up every ground I could find with a little sandpaper and it doesn't do it anymore. My alternator still sounds like that, though.

 

[Mojave_Z71]

I had a similar whistle. Replaced alternator and it did nothing. Turned out to be the bearing going bad on the water pump. New water pump and my whistle is gone.

I was able to diagnose it being the water pump by spraying a little wd-40 in the weep hole on the underside of the pump. That temporarily quieted the whistle down until the lubricant was gone. Only replacing the water pump finally got rid of the whistle.

 

[derekjl]

I had a similar whistle. Replaced alternator and it did nothing. Turned out to be the bearing going bad on the water pump. New water pump and my whistle is gone.

I was able to diagnose it being the water pump by spraying a little wd-40 in the weep hole on the underside of the pump. That temporarily quieted the whistle down until the lubricant was gone. Only replacing the water pump finally got rid of the whistle.

Interesting info. Did it also only happen at idle like I am describing?