Check Settings > Sounds and see if the melody matches any of the Sounds set there.
Some apps can set their notification sounds under Settings > Notifications > (the App). The Mail notifications found here can set a different sound for each email account. (If sounds settings are shown both here and in Sounds settings, they should be the same both places, if not, the Notifications will 'probably' override the Sounds setting).
Less likely, because of the frequent repeats, you can also check the Clock app; the Alarms and Timer can be set to various sounds.
Even less, less likely you can set custom ringtones and text tones for each entry in your Contacts app.