@wilhelm1
apparently you are running a diff. OS
Nope. As you have posted in a Windows 7 forum I assumed that your question had to do with Windows 7 OS, although the three steps I describe should pretty much work from Vista to Windows 10.
want to buy a thumb drive to use as a backup any suggestions as to the size I will need?
The size will likely be determined by the amount of data you will need to back up. So you'll need to get a sense of what you are dealing with data backup wise, then the software you will use to perform the backup and whether or not it supports compression (most do, some do not), and let that be your guide as to the size of storage you will need.
I would suggest however that you might consider getting an external spinner, something like this
Amazon.com
You'll likely get more bang for your buck and will be able to store multiple versions of backups before running out of space.
Back to the clicking noise...
I'm not sure that it is "system sounds", but it is worth looking into just for your piece of mind, maybe even consider muting the speakers to see if it still clicks.
AND
You do know that you can force Windows Updates manually, using the Windows Update Control Panel applet?