ETC LED fixtures (apart from Selador Classic) are able to ramp their fan speed up and down depending on the amount of heat in the fixture at a given moment. A base level of heat is always present when a fixture is turned on, even when it is not outputting light, but the heat significantly increases as more LEDs are turned on.
Source Four LED Series 1, Series 2, and Desire D60 fixtures have the ability to set a fan speed directly at the fixture, either auto, slow, medium, or fast. In addition, they can be set to use a DMX address to control the fan directly. It's important to note that these fixtures will ALWAYS turn their fans on if they heat up too much, regardless of the mode set on the fixture, in order to protect the LED array from meltdown.
ColorSource PAR and ColorSource Spot fixtures increase and decrease their fan speeds depending on the heat in the fixture. Their fans are not controllable externally, and they cannot be set to maintain a particular level, unlike the Source Four LED fixtures and Desire D60 fixtures.
Taken from the above chart, the averaged worst-case noise levels are:
Classic Selador fixture : 28.1 dbA (18.3 dbA background)
Desire D60 fixture : 43.1 dbA (18.3 dbA background)
Source Four LED : 39.6 dbA (17.6 dbA background)
Source Four LED Series 2 : 40.1 dbA (17.6 dbA background)
ColorSource PAR : 49.5 dbA (17.9 dbA background)
ColorSource Spot : 39.6 dbA (17.5 dbA background)
Note that ETC Selador Desire D40 fixtures, and Desire D60X and D60XTi fixtures, do not have a fan - they are cooled solely through convection.