I'm having a weird whining noise on the highway. When I have been driving on the highway for a while, and I step off the gas, the car makes this whining noise. As soon as I pump the gas again, it goes away. It sounds like a vacuum leak, but I replaced all my lines to no avail. It did seem to go away after I torqued down on my exhaust/intake manifold bolts, however the exhaust manifold was replaced by a shop last year so I would be surprised if it already had a leak. It came back after two weeks, and I think I may have just gotten lucky with the timing because it happens so intermittently. It doesn't really seem to impact anything performance wise. Is there some kind of gas problem that could make a whining noise?
here is a video of the noise. Usually it will go away as I slow down, but on occasion, It's stayed around even when the car is in park. On these times, I open the hood and try to find the source, but no luck yet, as it usually stops pretty fast.
My only other guess is possibly a clutch fan bearing, but the noise will often change in pitch and intensity, so I think it is a leak of some kind.
It's really annoying me, so when I replace my injectors I am thinking about replacing the manifold gasket as well. Looking for other suggestions before I do so. Thanks

 

Not sure ...but here are a few ideas...
Is the air-train stock or are you running a scam-air ...I mean, cold-air kit? Check all the sealing to make sure there aren't any cracks or loose hose clamps on the air-side causing wierd air harmonics. I've also seen where the rubber can get folded 'under' itself where the hose clamp goes. It's easier to do than ya think. I'd remove the rubber snorkel from the air-box & TB to make sure 100% both ends are clamped down 'right' and snug ..to rule no air is sneaking by.

You can check the intake side of the intake for 'leaks' by putting a 2' length of hose on a little propane tank and wave raw propane around and under where the manifold attaches (unlit of course!). See if you'd hear the engine speed up. But it could be leaking intermittently like your video seems to suggest. once you get the maniolds off, also inspect the exhaust manifold for cracks. They always crack. Check the throttle body base gasket too. Probably not it, but since yer checking things... Crankcase ventilation too. You 'could' block off most of the vacuum hoses & Emissions, temporarily and drive it up/down the road to see if it still whistles. If not, then you learned something.

You can 'feel' the fan clutch for 'freeplay' and bearing issues rotating both cold and engine hot. Doesn't sound like that though ..like you say. But it's a free thing to check. Vehicle 'off' of course.

Either way, it's not that big of a job to replace the manifold gaskets. If nothing else checks out ..it's not a waste of time. I mean it's more effort than changing a tire, but less than a heater core ...lol.

Also replace the exh man doughnut gasket unless it's pretty new. Check all the exh over real well. If the exhaust manifold is cracked, I'd have it welded. I've bought a few and they always ended up cracking. The ones I welded the cracks on, they held up better/longer. You can buy new exh bellows too off ebay if that's rotted away. Or a new exh man if you need one. But yeah, sounds a lot like an intake leak. You have good ears!!

Let us know what it is...

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Well, I couldn't hear it in your video.

Based on your description, I'd say you have an intake leak. Could be the intake manifold, could be your throttle body.

When you back off the throttle, the intake sees a very high vacuum. If there's even a small gap somewhere, it will pull air through it and the gasket will act as a reed causing the whistle. Once the vacuum reduces (hit the gas again, or go to idle) there's not enough pressure differential to force the air through. The intermittent aspect is likely due to the heating and cooling cycle of the engine where the gap closes or opens due to the different expansion of the cast iron head and alum intake.

What exh manifold brand did you get? When I did my '88 some years ago, I got a Dorman (old one was cracked). When I finished the install, I developed the whine about a week later. Tried retorquing but didn't fix it. When I pull the manifold off again, I could see burrs and a bit of weld "splatter" in spots on the gasket surface. I must have missed it when I installed it. After cleaning it up and making sure it wasn't warped, I put it back on with a new gasket. No more whistle. The whistle on mine was a pretty constant pitch, it just got louder or softer based on how much vacuum the engine drew.

Some of the bolts on the bottom are tough to get to, so make sure your torque everything down in proper sequence.

 

Could this possibly cause a whistling/whining noise? I had to fix a crack in it a few years ago, but it's worked fine since then, at least I thought.

 

No, the air isn't moving fast enough there to cause a whistle

 

Thanks. That's what I figured, but I thought it would be best just to make sure