Some G4 Mirrored Drive Door (MDD) systems have cooling systems that run loudly as the CPUs get hot, especially the model affectionately known at the "Windtunnel" G4s. On those systems, enabling nap mode brings the normal operating temperature down remarkably well, thereby quieting the fans. This was covered in this older hint.
The method is basically to install the last version of Apple's C.H.U.D. tools that allows Nap Mode on those machines, which is version 3.5.2 [21MB download]. That version still works with Leopard, although notably it misreports cache sizes. However, the old command line utility, and therefore the derived AppleScripts, that people have relied on until now to enable nap mode have broken.
So here is an Applescript I wrote that will enable nap mode again, even under Leopard: Ideally the next version of C.H.U.D. Tools would bring Nap Mode back for these models which need it. Merciful Apple, why have you taken it away?

