|Post back your exact acer model.|
Apparently some Acers have a very annoying beep that accompanies plugging and unplugging the power cord. I take it though it's only a single beep whereas yours is constant. Anyway the solution for that was to uninstall the realtek (I don't know if they all use realtek) audio drivers and let windows install generic ones. This however can affect other audio functions. Another solution is to upgrade to the newest realtek drivers.
When did this problem start with yours? I'm wondering if the 'new' charger is inadequate or isn't making a good connection. Does it beep when running just on the battery as well as when the charger is attached?
Does it start beeping like that before windows loads? If so it must not be a driver issue. In that case I'd suspect a heat problem, bad fan or problem with the charger or battery.