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My 2014 Mitsubishi Outlander (2.3L Diesel 4WD - bought 2nd hand 2017) started making a whirring noise a few months ago. Sound was diagnosed initially as wheel bearing so had one replaced. Noise halved in volume and only occurs at speed >40kmh. Definitely drivetrain related as only occurs under load. Mechanic said could be bearing in transfer case. Took to Mitsi for second opinion, diagnosed as transmission issue and now had a transmission fluid leak found. They quoted to replace whole transmission ($5000). I went for third opinion at transmission specialist as thought solution was a bit over the top. Diagnosed that gear box fine despite leak, could be transfer case.
I've now spent $1000 replacing a wheel bearing that may not have been needed and $300 on inconsistent diagnosis.
My question is - do I just replace transmission (second hand genuine part) vs attempt to find issue in transmission and get bearing (if that's it) and leak repaired??
Any one else experienced this and have advice?

Hi there,
Sounds like if the sound halved after a bearing replacement, then that was contributing to the issue, but not the sole cause. Which you may not know until elimination of all possibilities.
To repair the problem it may require a transmission overhaul, or 2nd hand replacement, but is it an issue you can ignore? The problem with used parts is that if the issue is common in that vehicle, there is no guarantee that it wouldn't also be present in the replacement one. Another drivetrain issue is that sound can transfer down a shaft and give the impression of an issue in a location it may not actually be in,. So at the end of the day, 100% problem solving may not be found until the parts are stripped and checked- but on an old vehicle the list of worn parts may be exhaustive.