I had the same problem. It started when my 2 week old phone went completely flat. I plugged it in to the same third party charger and cable I always used and it was recharging very fast with the wretched beep for every 1% of charge. Reading through the piles of similar experiences and dismissive responses of experts (must be a connection issue, no such feature) I switched to another non Apple charging block and the Apple cable that came with phone at 33% and it stopped. I then reconnected to the previous cable and charger at 43% and it started again. Pulling down the command center and watching I noticed that without touching the phone it would show a disconnection from the charger at each 1% then reconnecting with beep.
I then connected the third party cable to the other charging block and blessed silence.
So my surmise for my situation is:
1 My phone went flat and was charging as fast as possible (more than 1% per minute)
2 The usual charging port is a 30W so the phone is protecting itself by disconnecting every % to prevent overheating or some other issue when fast charging.
3 Changing to a 10W charger stopped the problem and the phone is charging at a lower rate. The phone was getting noticeably warm from charging the for the first time since I got it. Normally it’s never been less than 80% charged.
Hope this helps someone.