Just wanted add something here.
This is an inherent issiue which comes in both old alto and cultus. You must have observed that the sound goes away once car is warmed up.
Essentially, the gear box of both of these cars - especially the fifth gear - is placed slightly above the gear oil level inside the gearbox. On a cold start, the fifth gear is the part which is making that noise. Once the oil warms up, it rises to the level of the fifth gear and the sound goes away.
In conclusion, don't worry about it if the sound goes away after warm up. Only worry if it doesn't.