PowerBook Purring

I am writing to find out if anyone has come upon a solution about the “purring”/”crackling” sound that some TiBooks make.

There is a whole discussion going on for a few weeks on this forum but there has not been a response from Apple so far:

Apple Support Site: Discussions > PowerBook > PowerBook G4 Computers > Usage > purring Ti book . . . !

So far Apple has not acknowledge the issue or repaired any units (including mine) to my knowledge.