The security update that Apple issued a week and a half ago apparently caused some audio problems on Intel-based Macs, so the company has released an updated version.
Apple released Mac OS X 10.4.10 to fix some security issues as well as reliability issues with USB drives. It seems that version led to some Mac users hearing "popping" sounds, Apple said in a message announcing the update Monday afternoon.
If you've haven't already updated to 10.4.10, Apple has released a new version of that update (1.1) that fixes both the security issues and the audio problems. If you already downloaded 10.4.10, you're cool on the security front, but you'll want to download a smaller update to fix any audio problems, or to ward them off.
All three updates (the audio fix, the security fix, and a file containing both) are available here on Apple's site, or through the Software Update button under the Apple menu on your Mac. Power PC users need not bother with the audio update.