iPhone making weird alert-like noises

This problem has been driving my wife and I nuts for quite a while. My wife's iPhone (6s, iOS 11.3.1) has been making a weird alert-like sound. We already turned off all alerts, including the emergency alerts, restarted the phone, turned off all apps, reset the hardware, and the sounds still happen. They are loud, and they are not part of the standard set of sounds that the iPhone makes. The alerts are also intermittent, and they also happen regardless of whether the phone is on airplane mode or not. However, we never heard it in the middle of the night, which suggests that they are turned off by the "do not disturb" settings. I was wondering if anybody has a similar problem and found a solution. This is driving us nuts, as it has been going on for several months (I believe before iOS 11.2), and being quite disruptive sometimes. Help is appreciated.

Cheers,

Acacio