You're at 84K and haven't changed your gear oil? That's probably part of your problem. I've heard good things about redline, but I personally prefer Motul 300. The gears whine because they're straight cut, if I'm not mistaken.

 

It has had the regular maintenance done to it, they just have been putting the regular gear oil into it, and they recommended beefing it up by putting 90 weight in. I dont know anything about this type of maintenance.

 

Subaru recommends this? Or a 3rd party shop? Subaru shouldn't recommend anything besides Subaru's gear oil.

Mine has a little bit of a whine in 2nd and it has a recent fluid change to motul 300. If it was a diff bearing all gears would do it.

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Does it only happen in 2nd gear?
Can you shift early or late and see if it is speed related instead of a specific gear.
If the whine is in the differential it should not matter what gear you are in.
I have a GMC van where the rear differential whines loud 45-50 mph, but only there.
It is caused by gear teeth having too much wear and not lining up properly with the mating teeth.
The guy I bought the van from told me he pulled large boats with it and got a new one because
this 1/4 ton was not big enough.
Gear whine is telltale sign of harder use than designed for.
As for a fix, I replaced the oil in my van pumpkin and it did not make a difference.
Some day I will replace the gears.

 

The Subaru manual for the 2015 WRX recommends different fluid for the manual transmission than it does for the differential. It says you may use 75W90 but that it may cause problems. Redline makes a 75W90 "NS" non-slip that can be used for the MT, while the regular 75W90 without the "NS" designation can be used for the differentials. Since their are some Motul users commenting, has anyone put the Motul 75W90 into their manual transmission on a 2015-2017 WRX and, if so, what have been the results? I am just trying to figure out what options I have for the MT oil. Thank you.