Apple watch charging sound every 2-5 minutes

I used my apple watch for more than 6 months without any issue. Then, soon I updated the watch with latest software last month, it started to make ding noise every 3-5 minutes, and that noise started to make soon the watch reached hundred percent of charge. I took it to Geniuos Bar, sent it for repair, then it was replaced with a new apple watch by the local Apple Store, which I received day before yesterday. Since then, no noise came out from this new watch while on charge. All good. I was happy as I did not have to wake up in the middle of night for this ding noise while the watch was on charge. But today, as I updated it with latest software 5.1.3, and soon the watch finished update it made a ding noise while on charge. Thats the first one I heard. Now, I wonder was it a software issue again!! There must be some glitch in updated software. Lets see how many more times I would hear that annoying ding noise in upcoming days. If that goes on few more times then obviously I will need to book again with Genious Bar to have a look into it. Any other person’s experience in this regard would shade more light on this issue.