I have a couple of wireless charging pads that give me a problem. For no good reason, they stop and start and stop and start, beeping every time. Your problem is a little bit less severe, I think. I suspect what is happening is that, once the phone reaches 100%, it stops charging, as it's supposed to. However, the charger starts charging again almost immediately, causing the beep. The problem is almost definitely with the charger. The chargers I have that cause the problem are by Mophie. The Verizon brand one (not cheap) I have is not a problem unless the phone I manage to put the phone on it in such a way that it's not making good contact. Given that this is the one next to my bed, I manage to do that often enough. If I pick it up and set it back down, it's fine. If I don't, it behaves exactly as you're describing. The cheap, off brand charger on my desk at work works flawlessly. Go figure.