The account in What the Dormouse Said says the sound was intentional audio feedback:

"Engelbart referred to the on-screen cursor as a "bug" or a "tracking spot," and there were occasionally odd buzzing sounds in the background as he executed commands at the keyboard. The group had been experimenting with using the computer to generate different tones depending upon what was being executed, as a way of creating auditory feedback."

Looks like a pretty uniform buzz, too

http://i.imgur.com/BbaGsCu.png