Gearbox Funny Noise

[mik995px]

https://www.youtube.com/watch?v=W7l_lZgSBWc
https://www.youtube.com/watch?v=W7l_lZgSBWc

As can be slightly heard in the video, I have a funny sound coming from the gearbox (I think). it sounds like rolling rocks and is wheel speed dependent.

what I have done so far:

Raise the rear of the car, put it on jack stand and start the car.
On neutral, I dont get the sound even when revving.
I put it on First gear, you start hearing the funny sound.
while on first gear, I stop the right wheel from spinning (Open diff layf) to check if it is the right CV axle or wheel bearing. It still makes the noise.
I then stopped the left wheel from spinning. It still makes the noise.
I put it on 4th gear. the sound is now louder.

gearbox is a C52 I believe (plate in the frunk says so). 1985 Aw11 NA.

is this new gearbox time??

anyone have a spare in wellington area??

just for the record, I drove it to the October Manawatu MR2OC club run(28th October 2018) with it like that, and nothing happened. Feels fine. drives fine. all gear works.
Also, Im not very sure if its gone louder since I first noticed the noise ( been a while now. probably a month or so) since it gets droned by the noise from the engine.

[mknz]

Broken gear teeth?

Drop the oil and look for metal. At the very least it probably needs to be changed because nobody ever does it.

The good news is I saw a couple of gearboxes for a sale not long ago (where I can't recall).

[mik995px]

Ive change the gearbox oil last year. I did put a REDLINE OIL LIGHTWEIGHT SHOCKPROOF GEAR OIL in it. Some people said its not recommended due to its properties, but it did got rid of my 2nd gear crunching problem. So I kept it in there.

What gearbox oil do you recommend to put in these gearbox?

[mknz]

Well the BGB says GL-4/5 and 75/80W-90 but this topic is one where people swear by and hate the same oil.

When you say it doesn't happen in neutral, your foot is off the clutch?

If yes, then I think you should get a clean container to capture the fluid (just in case it's good and it's not the gearbox) but still check for metal.

I'm thinking that a tooth is broken and is being hit by one of the diff gears (so it's always rattling when one of the wheels is driven)

[MR2GUY]

On my E series I only use Castrol Syntrax 75-90, it's always been great to me and works wonders on synchros. I've heard motul is also good. Have used penrite with the nulon "smooth shift" additive in a friend's gearbox which seems to work great.

Is the noise still made while in gear rolling and with the clutch is engaged?