5th gear issue

[r.ave]

Hi Guys,

I seem to have an issue with my gearbox and especially with 5th gear.

I'm currently breaking in my rebuilt engine, so I’m not pushing the car at all.
When I started breaking in the engine I noticed I had a noise when the clutch pedal was released and the engine was idling. When pressing the pedal, the noise disappeared.
So i thought it had something to do with the clutch. Like a release bearing which isn't uncommon with other brands.
However, when listened to with a stethoscope on the gearbox, the sound remained to be there no matter what position the clutch pedal had.
We checked the gearbox oil, this was completely clean.
This noise only occurred when i had been driving for a while and everything was warmed up. Not when cold.

The car had been off the road in the winter so started driving it 2 weeks ago. With the previous in mind i kept a close ear if things got worse. It didn't.
However, at a certain point I wanted to overtake somebody on the highway.
I was doing about 3500revs in 5. I downshifted to 4, increased revs and speed.
I noticed the car was smoother, and did not have as much rattling noises of an old car.
So I started to checking a few things.
I came to the conclusion that the higher I rev in 5th gear (over 3500) i hear and feel some extra noises and unbalance and it increases the higher the revs go, and go down when the revs drop.
If I do that in 4th gear, at same car speeds and therefor higher revs i do not notice any difference with lower revs. Same as in 3rd, no difference with 4th.
Gearchanges in general are fine, up and down, no leakages.

The gearbox has to come off obviously, but would I be looking for just a bearing replacement, or a full overhaul?

Looking forward to what you think.