My phone keep making a very weird and alarming noise

So for ages now my phone makes a wired ringing noise at very first I thought it was a notification but nothing was on the screen to say so checked everything it was a message and then it kept happing over and over again I first happens with my previous iPhone I took it in a store the guy run and diagnostic on it and it didn’t come back with any thing after we reset my phone and the next month a I was dew and upgrade so I thought it would be fine when I got my current phone after a few days of getting the phone the same wired noise keeping ring on my phone again no notifications to got with it the the screen stay completely blank when it makes this noise and now I’m quite frankly tired of it , I’ve tried deleting all my apps resetting the phone and now it still happens o still don’t know what to do