this is from older linux kconfig diff:
+ tristate "Support for thermal management on PowerMac11,2"
+ depends on WINDFARM && I2C && PMAC_SMU
+ select I2C_PMAC_SMU
+ This driver provides thermal control for the PowerMac11,2
+ which are the recent dual and quad G5 machines using the
+ 970MP dual-core processor.
It means that linux have thermal control of Powermac A1177 inside OS.
So or linux (and MacOSX) controls the fans, or controls the SMU which do it independently.
And maybe MorphOS lack this direct control, so SMU behaves diferently.
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad