Hello Nael, this is just to let you know that the two posts you made in quick succession in the iOS8 forum have been deleted as they are a duplicate of your original post here. Doing that can fragment the topic of conversation with possible confusion as to where to answer you. And please note, it's against the rules to do so.
If you have tried to recognise all the sounds in Notifications, it may be a system sound. Perhaps you could provide further info: under what circumstances are you hearing this sound?
Regards, Andrew