I've got a '98 Chevy K1500 with a Vortec 350.

About three weeks ago, it started to develop a whistling/whining sound coming from the top of the engine, that sounds almost like an air leak. At that time, the sound would generally start after the engine had been running for about a minute, then go away after about ten minutes when the engine had warmed up.

More recently, the sound has been lasting longer. I don't normally drive more than a half hour at a time, and the sound has now started to last for the whole duration of those longer trips, while still starting only after the engine had been running for about a minute.

The sound is dependent on throttle position, becoming a slightly lower pitch as the gas pedal is first pressed, then becoming higher pitched, and disappearing as the throttle is pushed past about a third of the way open.

I confirmed this by manually throttling the engine under the hood - the sound reacts the same way, and disappears if I open the throttle too much, even before the engine rpm has increased very much.

I've checked vacuum hoses and they seem tight. The intake is also tight. I listened above the engine, and the sound is coming from the top of the engine, near the very rear of the engine compartment.

Any ideas on what the problem might be? It doesn't seem like an actual "problem" but it can be very annoying listening to it.

 

Well, sounds like an air leak. If it follows rpm, higher pitch at higher rpm. Probably intake gasket since it would quit after warm.

 

That's what baffles me a little, the pitch does not follow rpm. It follows throttle position, independent of engine rpm.

I know there is slow coolant loss, which can also be a sign of a bad intake gasket -

I'm not liking this as I've heard it can be a very time consuming repair that basically involves removing the whole top side of the engine.

 

I don't understand.

 

The pitch is related only to how far the gas pedal is pushed in. If I let up on the gas all the way and the engine is turning 1000 rpm, it sounds the same as if I let up on the gas all the way and the engine is turning 1500 rpm. When I push in the gas, the pitch of the sound actually goes down at first, before becoming just slightly higher pitched - The sound never actually increases more than a musical half tone before it disappears, it does not seem to be directly related to engine rpm.

Interestingly enough, when I push the gas pedal in about halfway to the floor and farther, the sound is no longer present.


I've noticed that the sound also persists for a second or two after I shut off the engine, with increasing pitch right before it stops (after the engine has already stopped turning).

 

Ok, so it does change with rpm, but there is a reange of rpm it dosen't. I'd suggest an intake manifold gasket still, and based on it getting better when warm. That is not as difficult fix.

 

This sounds more like the upper intake is cracked. They are made of a composite material and crack from the constant expansion and contraction. The short runs that you do also do not allow for the full expansion cycle...I saw many of these when I worked at the Chevy dealer.

1998 Chevrolet K1500 Engine Intake Manifold - Delphi, Dorman - TheAutoPartsShop - TheAutoPartsShop

The lower intake gaskets were more of a leak at the rear of the motor, not so much a vacuum leak or running condition.
This would also make sense that the 'whistling' is from the upper part of the motor

 

Interestingly enough, I did not hear the noise on my half hour drive home today. I will observe it tomorrow.

I'll be making about a 5 hour trip this weekend, and will observe it more then.

Thanks much for all your input.

 

What was was the verdict on the noise? I have a similar noise coming from the throttle body. After warm up and at idle it whines like crazy when given gas it goes away then comes back at idle or lower rpm range. Almost sounds like a pump or something.

I took off the belt thinking it was from a rotating part but the sound continued. Took of the air filter and cover and it’s definitely coming from the throttle body area.

 

I changed the IAC but the whine got worse, but the engine ran better, much more responsive.
If you put a stick to the ear to the black box on top of the
throttle body behind the bore and under the resonator, the whine seems to be coming from there very loudly.
This just recently started on my 99 Suburban,
so need some info on that black box. Mine also seems to go higher after killing the engine, strange.