Samsung Galaxy Note 10.1 — Pinging noise

Hi, I have seen this question on other forums, but have not seen any helpful answers. I am a Note lover. My husband and I bought the first two that became available a few years ago. Love everything about them. So when it came time I got a Note 5. I keep a lot still on my old phone and use it with wifi, since the Note 5 does not have an SD card slot.(The only drawback so far.) So now we have the Notes in our bedroom charging at the same time and out of nowhere they all start pinging. It was driving me nuts even figuring out where the sound was coming from as I have a Tab S and a new Gear S2 and a Poloroid tablet as well.
While it is odd that they all started pinging about the same time, they don't do it at the same time. Any ideas, cause I don't have a clue?