307 transmission sounds

My friends car 2004 1.4 HDI xr. I get bored when the engine or gearbox (I don't know which one from now) warms up, while the 2nd gear is watching 2500 rpm, there is an abnormal sound from the gearbox. This sound does not come when the engine is cold, it comes after warming up. Does this sound cause a problem?

Friends have no idea?
May your oil be blessed.

If there is a bndere in the same vehicle, if there is a buzzing sound, it is in the normal bnd but check the transmission oil.