Hi. Well I have at last established where the horrible noise is coming from. As suggested by a group member I put a thin strip of paper in the fan vent. The left hand side fan did nothing and feels like there is no air coming out. The back one, the fan stopped but the noise continued. The right hand side fan stopped and the noise stopped. As soon as the paper was removed and the fan started again so did the noise. So it looks like the left fan is broken and the right fan is very noisy. So I will need to replace them both. Do I have to use TiBook ones or will others fit? Simon Royal --- Site: http://www.simonroyal.co.uk. Twitter: http://twitter.com/SimonRoyal . Skype: Simon-Royal. (PowerBook G4 867Mhz, 1GB RAM, 80GB HD, SuperDrive, Mac OSX 10.5)