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I recently purchased a low-mileage '99 4runner, with a manual transmission and part-time 4wd. From the beginning, I've noticed a whine in gears 1-3, that increases in pitch when I'm accelerating. This whine is most pronounced in 1st gear; about the same in 2nd gear, possibly quieter in 3rd gear, and undetectable in 4th and 5th (comparisons all done at about 1500 engine RPMs, while moving.) The whine is also present while the transmission is in neutral, unless I've got the clutch pedal pushed down.

If this vehicle wasn't 4wd, I'd be pretty certain that the source of the whine was the transmission (which, if I'm not mistaken, is a Toyota R150F.) But as I'm about to describe, the transfer case is muddying those waters:

  • I hear the whine in H2, H4, and L4. They all sound pretty much the same, and the whine ramps down to nothing pretty quickly after I depress the clutch pedal.
  • The whine becomes less loud in t-case neutral. And when I press in the clutch pedal, the whine takes about 2-3 times longer to ramp down to nothing than it did for H2/H4/L4.

That last bullet above really surprised me. If the transmission is in neutral, then my expectation would be that the transmission's output shaft would be stationary, and certainly not driving anything in the transfer case! But I have confirmed that components in the t-case are spinning when the transmission is in neutral: if I attempt to shift the t-case from N to L4 when the clutch pedal is up, the t-case gears will grind. I have to wait for the whine to coast away to nothing before I can enter L4 from N. With the transmission in neutral and the car stationary.

My head asplode! But, aside from the whine, the vehicle performs wonderfully.

Assuming that this isn't just normal operating behavior for this equipment-- and I don't know that-- what other tests can I perform to isolate the cause? (I'd like to avoid removing the transmission, for example, if the problem is all in the transfer case.)

How do transmission specialists track this sort of thing down, before tearing vehicles apart?

I'm hoping to root-cause this problem before I talk to a transmission shop.

P.S. I've done some googling for this issue, and I have found some discussions. But they don't really end satisfactorily. Just suggestions w/ no follow-up (such as "try replacing your gear oil with synthetic"), or that the vehicle is operating as designed ("It's supposed to sound like a school bus".) Since this vehicle was already 17 years old by the time I bought it (but less than 60k miles,) I don't really have any mint-in-box reference to compare it to, as far as the operating-as-designed claim is concerned.

  • Can you compare the sound with another similar model? As the caretaker of a 43 year old 4wd, they have a vast array of sounds. – Criggie Sep 23 '16 at 5:14
  • Not really. This happens to be my first ever 4wd vehicle, but at this point I know that the whine is coming from the transmission, not the transfer case. As for the whine, it sounds exactly like bearing whine, like you might hear in a failing alternator. I know for certain that the input shaft bearing whines. The bearings for the individual gears may also whine, but they each sound different (except 4th, which has no whine. It's a 5-speed.) – Ryan V. Bissell Sep 23 '16 at 5:21
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    Not an answer, but do consider joining a local 4WD club. Much learning to be had, and the others can pull yours out when you get stuck. If you don't get stuck, you're not learning. – Criggie Sep 23 '16 at 7:34
  • Heading to my first event, 3 weeks from now. :) – Ryan V. Bissell Sep 23 '16 at 20:35
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If you get the whine with the main box in neutral, then the noise must be coming from the main box because the transfer box will not be turning. If you put the transfer box in neutral and the main box in gear, then there will be more momentum in the gears to keep the main box spinning longer when you press the clutch, hence the whine takes longer to fade.

Have you checked the main box oil?

  • Regarding your first sentence: I think you might have missed my middle paragraph-- the one after the 2 bullet points. I have evidence that the t-case guts are spinning (i.e., I can grind t-case gears) when the main box is in neutral. I am not claiming that makes any sense (hence my 'my head asplode' exclamation) – Ryan V. Bissell Aug 23 '16 at 21:05
  • Regarding your second sentence: the main box was not in gear. (And I intend to replace the gear fluids this weekend.) – Ryan V. Bissell Aug 23 '16 at 21:07
  • They will turn if the transfer box is in neutral, because the clutch plates will have a very slight drag. However if you put the transfer box in gear, the transfer box will not be able to turn, otherwise the car would move. – HandyHowie Aug 23 '16 at 21:10
  • In this scenario-- the one where I can unexpectedly grind the t-case gears-- the clutch pedal is fully up, so the clutch is fully engaged (not 'slight drag'). And, the transmission is in neutral. But! Maybe there is slight drag between one of the gears and a synchro ring, not enough to overcome the inertia of the vehicle, but can spin the input shaft of the t-case. ... And possibly, that could be due to low main box oil? What do you think? – Ryan V. Bissell Aug 23 '16 at 21:15
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    When in neutral, the gears can be 'slipping' on a spinning shaft. The viscosity of the oil can cause then to turn when there is no load to stop them. This can cause a very limited drive to the transfer box when everything is in neutral, hence the grinding. – HandyHowie Aug 23 '16 at 21:19

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