imac 5k sadly has a very active fan, especially in the combination of i7 and the amd r9 m295x gpu (4gb).
check the forums (wow, cod etc.) where many people are currently complaining and returning their mac (me as well considering it).. while gaming the fan speed is at 2700 rpm which is around 38 db (very loud!) and the gpu is 105C (not fahrenheit!).
all processor (cpu) intensive tasks like handbrake, rendering etc. will make the i7 go warm and the fan go loud.
all graphics (gpu) intensive tasks like any game will make the amd m295x go hot and the fan go very loud.
regular surfing, photoshop etc. is not affected!
in normal mode, it's always at 1200 rpm where it is not audible to me.
if you want it as quiet as possible go with the i5 and the cheaper graphics card (290x).
just be aware that this is the loudest imac or even mac ever (louder than current mac pro) and I'm not sure if constant heat (105C) is affecting life expectancy and I've been having them since 1991.