I had this problem when I was changing LCD and screen.. I had the LCD only plugged in and also had not originally unplugged the battery as I changed LCD's.. So, it wasn't lighting and was doing this beep (I also noticed the power button wasn't functioning properly when clicking to lock). I unplugged and then reconnected the battery.. The LCD then lit up as I powered it, but when I plugged in the charger, I still had the beep coming.. I then repeated the same steps above, but also connected the screen/digitizer.. Powered it up, and no more beep (and power button functioning as it should)..