HTC One M7 : How do I get rid of this weird notification sound?

[AC Question]

Hello. My htc m7 has started to give me weird notification sound when I'm browsing Internet every few seconds or sometimes minutes and don't know where it comes from. Started to happen when I traveled out of country and put in another sim card, now that I'm back, it's still on. My thought was that it was data roaming sound but its set to off, and have no idea what else could it be.?