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I have a 2004 Nissan Frontier 4X4, original owner, 240,00 miles, and running very good. In order to replace the valve cover gaskets I had to remove the upper plenum. I used a Fel-Pro gasket for the plenum. After completing everything I noticed a high pitched (almost electronic) whine coming from the engine (warm) while idling in gear with the brake on. If I just touch the gas the sound goes away. It's so annoying especially when stopped at a light or at a drive thru. I had replaced all the plenum hoses, and vacuum lines. I ignored the sound for a few months and decided to try replacing the gasket again last week. I used a Nissan plenum gasket (metal). The noise is even worse now. It still goes away once I accelerate. Should I try using sealant the next time around? Any suggestions would be appreciated. Thanks, Steve
 

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Get a can of starter fluid. With engine running, spray a little shot of the starter fluid in different areas around the plenum, if it is sucking air, the engine will speed up when it sucks in the starter fluid, sounds like its sucking air to me.
 

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Thanks. I'll try that. If it is sucking air should I replace the gasket any differently than the last two times? The surface is good except that I noticed a couple of very small gouges or indentations in the center of the manifold. Not on the edge. I have a new Fel-Pro gasket. I used new bolts the second time so the bolts are pretty much new. Sealant?
 

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While the engine was running with the e brake on and in gear and the noise was obvious, I sprayed starter fluid around the edges where the plenum meets the gasket. There was no change in the engine speed. I couldn't spray along the inside edge of the plenum. Maybe the leak is too small to affect the engine speed?
 

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Check your vacuum lines, hell spray them too, to me it sounds like vacuum leak.
 

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After more reading of your original post, you should check around the power brake booster also.
 

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While the engine was running with the e brake on and in gear and the noise was obvious, I sprayed starter fluid around the edges where the plenum meets the gasket. There was no change in the engine speed. I couldn't spray along the inside edge of the plenum. Maybe the leak is too small to affect the engine speed?
Take the serpentine belt off and run it like you just described but without using the starter fluid, this will eliminate power steering pump and alternator, both can make what sounds like a "super charger" high "whine" noise, it shouldn't take but a few seconds with the belt off to know,
 

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I have a 2004 Nissan Frontier 4X4, original owner, 240,00 miles, and running very good. In order to replace the valve cover gaskets I had to remove the upper plenum. I used a Fel-Pro gasket for the plenum. After completing everything I noticed a high pitched (almost electronic) whine coming from the engine (warm) while idling in gear with the brake on. If I just touch the gas the sound goes away. It's so annoying especially when stopped at a light or at a drive thru. I had replaced all the plenum hoses, and vacuum lines. I ignored the sound for a few months and decided to try replacing the gasket again last week. I used a Nissan plenum gasket (metal). The noise is even worse now. It still goes away once I accelerate. Should I try using sealant the next time around? Any suggestions would be appreciated. Thanks, Steve
Manifold vacuum is at a max at idle. When the throttle is opened, vacuum drops. So it appears you have a tiny leak somewhere. The gold standard method to find said leak is a smoke test which can be only performed by mechanics as they have the smoke machines.
 
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