I've recently bought a new macbook pro 13 retina (early 2015) and I've noticed that when the fan stops it causes a subtle but noticeable "click" sound. I was able to reproduce this with the yes stress test. To test this type yes > /dev/null & yes > /dev/null & yes > /dev/null & yes > /dev/null & in the terminal, wait for the fan to start working and then stop the processes with killall yes. After a few seconds (I suspect when the fan is slowed down to the certain speed) a subtle "click" sound appears. Is the fan designed that way, so it doesn't spin unnecessarily? Or should I consider returning the unit? Thank you for the responses.