When you lock a keychain in OS X, the system will make a brief sound like a latch closing, which indicates that the keychain has been locked. In most systems this will not regularly happen, but if you have created alternate keychains or have modified your login keychain to automatically lock, then this sound is a convenient indicator that you will need to reauthenticate to use your keychain. A number of people have found that after upgrading to OS X 10.6.7 they hear this keychain lock sound occur rather frequently.
This problem is likely happening mostly for people who use the popular password manager 1Password, which appears to have been a problem with the beta-10 release of the software that people might have been using. The developers have addressed this problem and suggest the following options for people who are experiencing it:
Downgrade to the latest stable release
If you are running the beta version of 1Password, downgrade to the latest stable release (currently version 3.5.9). This can be obtained from the 1Password Web site.
Install the latest beta
Agile Web Solutions has updated the beta project with a new build that should fix the problem. If you are still interested in running the beta version, be sure to update to the latest release.