The correct gear oil for Khyber/Cultus is 75W90 GL-4. When my Khyber's gear box started producing humming noise, my mechanic advised me to pour a thick gear oil additive. I knew it was not going to work but upon my mechanic's insistence, I poured it and it didn't work. Then I changed the gear box and poured in the correct gear oil (ZIC). It worked soundless for a year or so but it again developed the same humming noise.
When I was changing the clutch/pressure plate of my car, I saw a very clean gearbox of Cultus. It was of 2013 model cultus (PKDM) which was accidented pretty badly but engine and gear was not damaged at all. The shopkeeper was asking Rs. 15,000/- for it. I purchased it for Rs. 12,500/- when all the other gearboxes were costing Rs. 9000/- This time I used the KIX gear oil 75W-90. The junkyard guy gave me two weeks guarantee. I checked it thoroughly during that period and there was no noise at all. But the humming noise again appeared after two months or so.
My advise (which I learnt the hard way) is you should keep driving with your current gear box, just pour in the correct gear oil, the humming noise will stay, use the music player to distract yourself from this noise 
