My car makes a similar noise and I'm 97% sure it's my rear differential. Although it could be a very slight noise from the transmission. Mine happens at around 3k rpm in 6th gear with extremely light throttle application.

 

Check the balance shaft..maybe bearings its fed NOT tcase bc even if its neutral it would whine and cluct in or out..so check ur balance shaft..could be u don't have enough transmission oil..did u change ur clutch recently?