I had a perfectly quiet MacBook. Contrary to the MacBook Pro of my friend i never expierenced any hissing sounds when idle or suffered from always running fans. This is history now after the recent SMC update of Apple. For keeping the macbooks cooler they tuned the fan settings to effectivly keep the fan running all the time, at least with a mininum RPM. Whether the hissing sound is in direct relation to the fan speed i cannot tell yet. We are still speaking of a very low volume noise level here. You can't hear it at work in the office space so it took me a couple of days to realize this new reality, i just did not want to accept the truth. Now when stitting at home, sunday morning, quiet, coffee, i keep hearing it sizzling along. I would prefer my macbook getting hot instead of noisy, especially in the now upcoming berlin winter.

have fun