I agree with it being a faulty charger. My phone beeps continuously if I use either of my Samsung wall chargers, but I can use my husband's Motorola or Verizon chargers with no problem.
I have a Galaxy S7 with a Wireless Fast Charger which was recently upgraded to Nugget operating system. After the upgrade, the charger began beeping every few seconds when in the charger cradle. It was driving me crazy. After playing around for an hour or two I found that if you TURN OFF SCREEN SAVER it stops the beeping. Settings > Display > Screen Saver > OFF. Hope this helps.
My phone suddenly starting beeping randomly, the same beep you get when the charger is a bad fit or the contact is otherwise bad. Assuming the socket might be dirty (as with most things you shove in pockets) I used a contact-cleaning spray on it, and for the time being this seems to have solved the problem. It is possible, however, that I might have to get this socket changed/repaired.