Typing using a microsoft keyboard. Using a lot of the left side letters but also the space bar, enter and sometimes the numpad. There is a really low volume ambience sound.
Inserting 3. 5 inch floppy-disk into a floppy disk drive, with the typical sliding sound first and then the spring-activated locking mechanism snapping it shut.
The relentless sound of a data server whirring away in the corner of a busy solicitors office. Never switched off, this hypnotic and maddening sound is an omnipresent feature of the modern office environment.