I did go out and buy a replacement drive (LG 24x) and, while it has yet to automatically dismount, also makes similar clunky noises. It's only the top drive. My Mac Pro has two optical drives and the bottom one is constantly silent. I bought the Mac Pro with one drive but added the other. From day 1 I suspected a flaky drive, but I didn't expect the replacement to make the same noises. Though I'll admit the noises don't occur as often and they aren't as loud... Is it normal for Mac Pros to periodically check for any discs in the drive? Thx for your input!