Thanks. It was the Windows "asterisk" sound. It seems, however, that NoteTab might have a way to be configured so as not to trigger that sound when the cursor hits eof. Someone (not I) might not want to turn off something in Windows.
Pressing down arrow or right arrow at the last character in a document makes an annoying bell sound, but I cannot find anything in the documentation about sounds, or how to configure for no sounds. Thanks :-)