Gearbox Noise

[InkWells2]

There is an annoying whining noise in 1, 2, 3 and 5th gear particularly when under accelaration. 4th gear is incredibly quiet. This specific noise in gears had been mentioned in an earlier post some years back . The gearbox was a replacement put in just 20,000 km ago by the previous owner of 12 years who maintained the car very well said that the noise was there when the box went in and it hasn't got any worse? Why would 4th be quiet and the others noisy? I imagine big $$$ to fix.
Regards Ian

I received one suggestion when I posted this with my introduction but thought I may be better asking the question here.

Dales 300exc posted this 4 me ......
My gearbox is the same and has been for 100,000 km. 4th is quiet because it is direct 1:1 ratio and doesn't put load through the countershaft like every other gear does

[Parksy]

Dales300exc's explanation is correct. Basically, when in 4th gear, the input shaft is directly connected to the output shaft, so no gears meshing together, where as every other gear has the drive from the engine transferred through the lay shaft.

As for your gearbox noise, have you changed the oil?

[InkWells2]

Thanks for your response ...Yes I changed the oil... just a day or two ago .....based on a suggestion from another member I used a 50/ 50 mix of 75-85 gear oil and DX III auto transmission fluid, I also added a $25 tube of NULON G70 gearbox additive which is meant to reduce wear & noise etc also . I can't say IO noticed much improvement maybe a little. I also notice that when I am sitting still in neutral there is quite a bit of noise also ... and the noise reduces a lot when you put the clutch pedal in... Could that be a throwout bearing ??or something like that and could that also be part of the noise in 1, 2, 3 and 5. The car had a new clutch at the same time as the exchange box about 20,000k ago.