Q: iPhone 6S Plus keeps on making alert sounds

My iphone 6S Plus keeps on making alert sounds even though there is nothing to notify. No messages, no emails,

nothing on the notification center, no badges.

I have tried turning off notifications, holding the home and the power buttons at the same time...nothing seems to work.

Only the do not disturb  button seems to offer some peace of mind.

I had never experienced this before.

Have anybody experienced this same issue? Could this be a bug?

Any suggestions or advice?

Thank you in advance.

P.S. Problem persists even with the do not disturb button on.