This sound is caused by Windows trapping the Alt key, because Windows reserves the Alt key for use in menu selection. Since SWTOR has no menus, it produces the Default Beep that others have mentioned.
Unfortunately there is no way to tell Windows not to trap the Alt key. One way to resolve this would be for SWTOR developers to modify their code such that the SWTOR game client contains a hidden menu that allows Windows to correctly trap Alt but do nothing, hence no beep. Many games do this, which is why most people haven't seen this problem in other games (for example, LOTRO doesn't beep, but AOC does).
For more code-geeky information on this topic, check out this link:
http://chrisprogrammingtips.wordpress.com/category/annoying-windows-beep-on-alt-key/