This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
This is a problem primarily with the RiscPC machines caused by the fan lubrication drying out. Symptoms generally include the fan making a lot of noise when the machine starts and then, once warmed up, the noise going away. Progressively the amount of warm up time gets longer and longer.
The solution, and this should only be attempted if you are confident in handling your machine's internal components, is to strip the machine down to it's base slice. Remove the label from the fan and the rubber bung from the bearing. Place a drop of three-in-one oil (and not WD40) on the bearing. Replace the bung and reassemble the machine, your problem should be cured.