Huawei P20 Lite shimmers after closing apps?

My mother's smartphone (P20 lite) is always buzzing with a message tone after which apps are closed. Example: It opens the browser or the messenger, searches or writes something, closes the app and about 3 seconds later a message tone rings. I've tried to take care of it, I've gone through the settings countless times, also under more sounds, lock screen, have the message sounds completely issued and turned on only for certain apps… No matter how I have set, the message sound comes on closing apps. Since I personally no longer know, I hope to find someone here who may have a final solution to this problem

If you have durchexerziert all the settings, I would just think of a reset of the phone in factory settings.

That would really be the last option. Before I do that, a response from someone who has fixed this problem would be really helpful.

How old is the smartphone?

Had with me the case that there were sounds uncontrolled. In the final defect was the battery, as strange as it sounds.

Battery, I think exclude, because the problem does not occur sporadically and the phone is half a year in operation and is operated only with original accessories. Nevertheless, thanks for the thought-provoking.