Autotest, which is part of ZenTest, is a very handy testing application. It runs tests as changes are made to the code. When using it, I would accidentally leave it running and then notice something using up CPU cycles. It would turn out to be the autotest process that was still scanning files for changes every so often. I would then stop autotest only to be bothered to start it up again when I was working on the project again.
Awhile ago, I ran across autotest-fsevent for the Mac. It uses the Mac’s FSEvent core service to determine which files have changed (or to be notified when they are). This appears to have really helped the CPU cycles especially when nothing has changed. It is immediately notified when a file has changed.
I would also recommend upgrading to the latest autotest-growl as well. I was using an older version and there have been improvements.