Reading back through some older threads on G4 iMac's, I came across this comment, "While you have the unit totally open and drive bay removed, vacuum/blow out the years of accumulated dust. Use a pencil to hold the fan stationary so the air blower doesn't over-spin it." Several times with hard drive & optical drive removed, I have cleaned the internals of disgustingly dirty iMacs with a high pressure air supply (not a pressurized can), and recall that the fan inside at the top has spun remarkably quickly. I'd then remove any residual dust from individual fan blades using a soft brush. My question is simple. Could these (and similar fans) overspin, with a subsequent drop-off of efficiency? And what adverse affect (if any) if a fan ran 'backwards' during hi press cleaning - as I'm sure all mine must have at one time or another. All four of my G4 iMacs have been thoroughly cleaned internally with high pressure supply, and all seem to function ok. However the fan of one 20" runs fast (is notably more audible) immediately following boot, and is the same during functioning. The other is barely audible. I'd be interested to hear comments and experiences of others.