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.