5th Gear Whine - Rav4.2 D4D

[ShinraMoogle]

Hello all!

Driving a 2004 Rav4 2.0L D4D, 163k miles.

Recently developed a whine (sounds like putting your finger against a rotating desk fan if you know what I mean) only in 5th gear at around 50-70mph, not a single noise in any other gear except 5th. Seems to be mainly when putting my foot down to overtake or when I lift my foot off the accelerator. But as said if I was doing 50-70mph in 4th gear there is no whine.

I know 5th gear usually does fail on these but its still going into gear perfect and still has full power, would old filthy gearbox/transfer case oil be a cause of this or would it just be 5th gear on its way out?

Any info appreciated, thanks!