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The Noise from Hell

This is an ongoing problem that has lasted for several weeks now – I’m completely flummoxed and at a loss. It goes something like this:

My truck (2000 Dodge Dakota, 2.5L, 5-speed) has developed – for lack of a better term – a high-pitched whining squeal, which only occurs after I put it in gear and actually start moving (if I leave it idling in neutral, when starting it after letting it sit overnight, the sound usually does not appear at all). After about three or four minutes of driving time, the sound will show up, but there’s an odd twist: the sound is most apparent, and really loud, when the truck is at idle, such as when at a stop sign or traffic signal; when I put it in gear and take off, the noise will drop in pitch and, depending on how hard I’m pressing on the accelerator, it sometimes disappears altogether. When I take off, every time I shift gears the sound returns – only for as long as I step on the clutch and back off the gas, and when I step on the accelerator again the noise falls in pitch and vanishes. It does this off-and-on thing through all the gears, and after reaching the desired road speed (and I’ve reduced pressure on the gas pedal) the sound returns and is constant. Also, if I shut the truck off and wait just a few minutes, and then fire it back up, the sound will be gone, but will return after a short while, exactly as related above. It’s worth noting that, during all this irritating misadventure, the engine has run exactly as it always has – there’s been no discernible impact on performance.

Now, some considerations. The noise is louder and more strident when the ambient temperature is lower, such as in the morning. Later in the day, when the temperature rises, the sound becomes erratic and somewhat unpredictable, and is much lower in volume, and it will even stop abruptly and then reappear later.

Many people have suggested that it could be the belt or a pulley, maybe. I replaced both tensioners and even installed a new, shorter belt to bypass the A/C compressor (to eliminate it as the possible culprit), but the noise remains. I was skeptical of all that because – and please correct me if I’m wrong – a shot pulley making such a horrible racket should get louder as engine speed increases, not drop in pitch and disappear. In fact, the noise seems to have no relation to engine RPM; if anything, it seems tied somehow to intake manifold pressure (the sound vanishes when the truck is accelerating, which is when intake manifold pressure goes down, or so is my understanding), or throttle position, or something else entirely. The vacuum system? The noise seems mechanical to me, not like escaping air (or, well, vacuum) . . . but what do I know?

Please excuse my long-windedness. Like I said, I’m at a complete loss and being driven completely nuts by this. Any feedback on this nonsense would be greatly appreciated.

“…it seems tied somehow to intake manifold pressure…”

When it’s making the noise at idle, pop the hood, then move and squeeze every vacuum line you can reach. Also, listen closely to the power brake booster.

I think it’s a vacuum leak; maybe an intake manifold gasket.
Drive until it will make the noise at idle. Pull off the road and open the hood.
Try to locate where the sound comes from. You can use a piece of hose as a stethoscope.
Use some carb cleaner, starting ether or an unlit propane torch and spray around the intake manifold and vacuum lines to see if the idle speed changes.

Go out and try wiggling the radiator fan.

If you can wiggle the fan, the fan clutch is shot.