I have had a couple of these machines and both of them had the fan running most of the time, at least when plugged in. It seems that once they reach a certain temperature the fan just stays running at about 3500 rpm until they cool down significantly for an extended period of time. I'm not sure if this is true of all T420s but for the two I've owned it appears to be true. I also replaced the heatsink/fan and applied Artic Silver on one of them to see if that mattered but it didn't. On battery they seem to be relatively quiet but I think it depends on how much of the discrete (nVidia) GPU they are utilizing at the time. I guess you could consider it a design flaw or limitation based on the limited cooling capacity of the machine. I hope that helps!